Can I add return air vents to improve air conditioning in some rooms?
If you keep the doors to the other (warmer) bedrooms closed, try opening them. If this helps, then a return air duct, bypass vent around the door, or a door grille could help. 

If opening the doors doesn't help the other rooms, you can try adjusting the vents (open more in the hot rooms, less in the cold rooms) to balance the air flow.

Also check the windows &amp; blinds. If the warm rooms get more sun exposure, close their blinds or rotate them so hot air rises toward the glass. This can make several degrees difference during the day.