So, the maximum age gap between the 1st batch and 3rd batch would be 2 weeks, right? If so, you may be able to keep them together. Each time you get a new batch, use a screen to partition the brooder so the old and new chicks can see each other for a day. Then remove the partition (ideally at night under low-light conditions).
There will be some chasing and pecking, but they should be OK if they're in the same size class (e.g., large fowl). Monitor them to make sure everyone is getting along. Adding 4 to 4 is a much better scenario than adding 1 to 4, or 4 to 1, as the single chick then becomes a target. I would not recommend adding bantams to large fowl chicks, as the size difference is too great.
As far as temperature is concerned, you could hold the temp around 85 degrees for the 2 week period you're introducing the 2nd and 3rd batch.