I must agree with one piece of advice from Pamda ----- Take plenty of water!
We went to Chichen Itza in June a couple of years back. It is so packed into the jungle, there is little breeze -- unless of course you're at the top of Castillo!
You CAN see everything in 3 hours, but you probably won't get to spend A LOT of time in each area. Some of the places -- especially the Observatory and the Ball court -- you just want to sit there for a while and soak it all in!