We went through that phase, and it was tough. Most common strategies we used:
- 2 rooms, preferably adjoining. When adjoining rooms were not available and kids were little, parents would split up. I am sure that contributed to there not being a 4th kid.
- Suites. Embassy Suites worked out great for us, especially as the kids got into their hungry teenage years. The "free" full breakfast for 5 was a significant value.
- Apartments, especially in Europe
- Look hard for hotels that have rooms with 2 double beds and still allow a rollaway. Hampton Inns and some Choice properties seems to work most frequently. Space was tight, bathroom competition fierce.
- Cheat. I'll admit it, when the youngest was of crib age, we would occasionally bring our own portable crib and cheat in a 4-person room.
- Hook up with another family of 5. Our best friends also had 3 kids, so when we travelled together (10 total) we would get three rooms, the 4 boys of similar ages would share one room and became great travel buddies.