Three months old and wont go to sleep in crib

My girls will be three months old on Tuesday.  They had been sleeping in their cribs when they got home from the hospital. Then, one night they fussed and the only way they would go to sleep was in their carseats.  Baby A sleeps in her carseat in the crib and Baby B sleeps in the seat on the floor in the room.  When would you suggest putting them in their cribs and not in the carseats?  I am scared that if I lay them in the crib, they will wake up sooner then in the seats.  Right now, they both sleep until 5 in the morning.  Though our nighttime schedule puts them to bed between 8-10.  We have also gotten into the habit of putting them down for naps in their swings.  We usually wake them if one is up and then change diapers, possible play and then feed or visa versa.  When should we be puttin them in their cribs for naps?  Should I start with one nap a day in their cribs?