Putting baby down for naps

Any tips for putting my 3 month old down in crib for naps? Two times I’ve put him down he slept 3 hours. Sometimes he will sleep for 40 minutes. But usually I set him down and he wakes up immediately or after only 10 minutes. I’ve tried laying him down drowsy and he just gets alert once set down. I end up having to hold him for his naps and I don’t get any time to myself. Advice wanted! Thanks!