You're not alone. I think jealousy is honestly a natural feeling in these situations... Everyone throws it as a very ugly emotion (which is can be) and so when you feel it you also get this guilty feeling (assuming you have a conscience).

I felt it a few weeks ago and I know I felt so angry at myself for even allowing myself to feel that way... Which just makes the situation even worse... It took the people on here to help me understand that it's okay to feel it, it's an emotion...

One thing you need to work on though if you want to stay is going to be taking it out on your DH... All that does is drive a wedge... I know I'm guilty of it too. And sometimes you just have to take a deep breathe and go do a stress reliever. I use running, or MMA fighting, or he!! I've using throwing tires before. For me physical activity helps me cool off and think straight so I can have a more rational response. I know some people on here use creativity as well. You just have to find something that works for you and then go from there

if you’ve not read Stepmonster by Wednesday Martin, I highly suggest it.  

Also, disengagement is your friend.  Let Dad be the parent.  If he wants to feed his kid junk, well, it’s his kid.  I have good step kids in my life, and I still have so many errands to run when they’re here.  Embrace it!  

Because if she’s 3, you’ve got a long road ahead.

It is so unfair to him, as I take it out on him, rather than the child.

Nope nope NOPE! You are right to place your anger on Dad, because he is the parent who needs to correct the behavior but seemingly doesn't.

Kids will be kids. That isn't an excuse for their behavior, but it is the reason. It is the reason why parents havr to set firm boundaries and actually parent their kids - otherwise, kids will do selfish, stupid, and hurtful things as they stumble through life. They are programmed to learn through playing and observation, and it is the responsibility of their parents to set boundaries and provide context.

When she pushes herself between you and Dad, what does Dad do? Does he tell her to wait her turn, move her to the side, and keep hugging you? Or does he stop hugging you to give her attention? Same with her moving you off the couch and taking over your spot.

If your FH's immediate reaction isn't to tell her no, wait her turn, and finish what he is doing, HE is the problem. Kids want attention. They want to be selfish. They want instant gratification. Dad SHOULD be telling his daughter to stop, wait, no, and delivering consequences when it doesn't happen. If he isn't doing this, HE IS THE PROBLEM AND YOU SHOULD BE ANGRY AT HIM.

Your SD is 3. Toddlers have righteous attitudes. If Dad isn't correcting it, as is his job, then it won't get better. You can't blame a kid for being a kid (there are caveats as they age) if their parent won't guide them. Most entitled, attitude-filled, bratty kids are a direct result of poor parenting from BOTH parents.

It's not her fault she's alive and breathing. If her very existence infuriates you, you need to find a way to deal with those emotions. You don't deserve to live like that, but neither does she nor your partner. If you are starting out this way, with a three year old you have an unhappy life ahead of you. But this is fixable. Please find a therapist to help you explore these emotions and find better outlets. Jealousy might be a natural human emotion, but so is rage, so is resentment - you don't want to live your life steeped in those emotions. Do be careful though - I've heard many people on this forum say that therapists don't always understand the step family dynamic. It's a good thing anyway for couples to really think through their relationship before they tie the knot. 

If you think this is too expensive while you're planning a wedding, please understand that the rates for divorce lawyers are much higher. (I'm shelling out for one today!)

On the other hand, you may have some legitimate grievances and as you set up a new household together you need to get to the bottom of how you're going to run it as partners and that includes parenting his kid. I think a lot of resentment on this forum is driven by guys who just assume because you're a woman you'll take care of things in the 'womanly' domain - like kids. Kids who aren't yours. I am absolutely blessed by a being with a partner who never, ever lumbers me with his kids. Sure, as time has passed, we have a division of labour in our household that means that sometimes I do stuff for his kids as 'my job' (like meal prep, but he does washing up and kids clear table). However, I also bring a kid to the mix, so he does stuff for mine, too. 

3 yr olds are awful. Cute, but awful. They shouldn't call it the terrible twos because they're worse at 3. They're so darn cute because if they weren't we'd throttle them. They are developing a sense of self and autonomy at that age, but it is selfish and blinkered. It's our job to guide them into thinking about other people's needs - but their little brains really aren't able to process other people's perspectives fully. 

Yea, all these comments above mine are spot on.

I'd just add that a very long engagement is in order so you can see if your fiance will step up to parent better after you've talk over your concerns. More time gives you a chance to really think this over and be sure you want this for the rest of your life -- it doesn't stop when skids are 25 years old, contrary to what you'd expect.

My BS3 had a meltdown because Daddy hugged Mommy and wouldn't let BS3 sneak in the middle. BS3 is going through a very possessive phase of "MY MOMMY!". In that little mind, no one is allowed to hug or kiss Mommy except BS3. No one is allowed to hold Mommy's hand except BS3. etc. It's my job as Mom to teach him how to deal with his emotions and reassure him that I love him without giving in to that possessiveness because giving in to that behavior will hurt him in the long run.

This is normal behavior but it needs to be parented. BS3 gets plenty of affection, but he's not allowed to interrupt if I'm reading with BS6, helping SD13 with homework/crafting, playing video games with SS12, or showing affection with DH.

It IS right and fair that you are irritated with SO if he is not taking steps to manage the behavior. SD3 is not deliberately acting this way to hurt you. Or maybe I should say rationally - 3yos are still very instict/emotionally driven. I think you are misinterpreting the "looks of attitude" you are getting and attributing more rationality to her than she actually has. From my reading, your irritation is with your SO dropping you like a hot potato when SD3 wants something. That's the real issue - not SD3, but because she is involved it has turned into resentment of SD3 and it's easier to blame her attitude than admit your 'perfect relationship' isn't quite as perfect as you thought. Your SO is struggling to find the balance between parent and partner. 

If SD was 6/13/16 etc I would probably have a much different bit of advice. I think you have recieved a lot of really good advice from the folks here. I hope you and your DH are able to discuss things and work the issues out in a way that benefits you both.