Now that it's hot outside can 3 month old sleep in his room with the air conditioner on? What temperature? Also how should he be dressed for sleep?

Yes at nl temp. A 3mo has the ability to self regulate temperature within comfortable ranges. A room kept at comfortable temp range for a normally dressed adult is fine. (commonly 70-75) by 3m a baby should be alone in diaper, shirt & onzie on a firm mattress with no pillows, bumpers, blankets. You can tell if baby is a bit warm by feeling sweat at the back of the head, cold if you see chill bumps.