I wouldn't put the two kids that come over on a regular basis in one room and save an empty room for the kid that rarely comes over and will age out in 1yr.

Go with what is best for your financial goals. The eldest should have a room to herself regardless. The younger ones can share. The 17yo will likely move beyond regular visits within a couple of years. You can adjust at that time.

No need to specifically assign a room to any single kid. Have a kid room and a guest room/office if you go with a 3br option. Put a trundle/day bed in the office and your daughter can stay in there with some privacy when she comes. Since she rarely visits the guest room can be used by either of the other kids when the eldest is not there. Never call it anything but the guest room to avoid the "my room" issues.

This is the basic model we have always used when we have lived in 3br homes. MBR, Skid room, Guest room/office. It works great. My parents use a similar model only with many more bedrooms. Guest room 1, guest room 2, G-kid room. If both of their son's families are visiting at the same time each couple gets a room and all 4 G-kids stay in one room together. Currently the kids are 22B-mine/21G-niece/18B-nephew/13B-nephew. Even when one or more of the parent couples is not visiting when the G-kids are there, the kids insist on piling in their room together. They have a blast. This is the model that has been in play since the eldest two were the only two.

When we would have adult guests the Skid would move to the guest room and his room would become the guest room because he had a queen and the guest had two single beds in trundle configuration. When my brother's family visited us all 4 of the kids would pile in the guest room and we would add two aero beds to the two trundle beds already in the room. Nice and cozy and the kids loved it. The laughs were abundant when one would get up in the middle of the night to go to the bathroom and step on someone.

None of these kids in your situation are resident so as long as they each have a comfortable place to sleep with reasonable age appropriate privacy don't sweat this one bit though I would give the near adult a room of her own when she is there.

The first house we bought was 3br/2ba. We used the model above. Our second home was 6br/3.5ba and it was only DW, Skid, and me. No issues there other than having enough furniture for all of the brs. My SIL moved in with us for her freshman year of college. She changed brs three times over that year just because she could. One of our spare brs was our study. Next we lived in a 2br apt. The Skid's room was also the guest room and when we had company he moved onto an aero bed in our master closet it was huge and had A/C and heat vents so it was basically a very dark and quiet br for him until out guests left. The three of us had many laughs when he was sleeping in our closet. Next we moved down two floors into a 3br apt. Back the standard 3br model. Next 3br/3ba with a third floor loft/game room. The third floor was our guest room since it had a bathroom so our guests could have privacy. Of the course the kid wanted that room but that crappily built townhouse was like living in a sieve so when we did not have guests we closed off the top floor to keep from freezing our butts off in the winter and cooking in the summer and to save on duplicating the national annual deficit in energy bills. Great place gut the builder should have been lynched. Luckily it was a rental. Now we have a 4br/2.5ba place that is just about perfect. It also has a separate dining room and a separate formal LR/study so we don't have to use any of the brs as a study. 2 dedicated guest rooms, the kids room all upstairs and the MBR down. Guests can stay as long as the want and the Skis has a key and can visit anytime he wants. I would like it better if it had one more full bath upstairs just so guests don't have to be exposed to the SKids bathroom. :O The only mandate is that my mom has claimed the guest room at the back of the house upstairs that overlooks our lake front back yard. Don't argue with my mom. She gets whatever room she wants. :O

Flexibility is the key I think regardless of how many brs you have. The term "guest room" regarless of how many extra rooms there are alleviates the "my room" issues in nonresident kid situations.

Does it have a basement? Usually older s/kids love to have that kind of privacy.