Crib training is HORRIBLE!!!

My son is 3 months old now and i think it is time to get him out of his rock and play and into his crib. He wont sleep flat, or on soft surfaces. We bought a crib wedge and it wasn't high enough so we put a small pillow under the mattress and a boppy under the sheet to keep him from sliding down. He will only sleep for about 20 minutes at a time before he starts screaming and i know he is still tired because he falls asleep in our arms when we pick him up. So i am looking for advice on how to make this transition easier for us and baby. Thanks guys!