Why do you "need" to read him to sleep? Who is the parent and who is 3 yo? You need to set the rules, just like you did when you told him he needs to sleep in his own bed. Set a routine and stick to it. He will get used to it if you are consistent.
Example -
Bath
Jammies
Vitamins
Teeth brushed
Read (2-3 books or a set amount of time)
Tuck in - Good Night
Mom and Dad are then done. Son stays in bed and eventually goes to sleep. Our son goes through spurts where he tries to call us into his room after we tuck him in - for various reasons (all a ruse to just get us in there). We say "It's nigh-night time. Good night. Love you." - through the door. We don't even go in again unless we know he actually needs us. Sometimes he gets upset for a couple of minutes, but he knows that's just the way it is. He is asleep 10-45 minutes after getting tucked in. Yes, sometimes he talks in his bed, look at books by nightlight, or plays with the couple stuffed animals in his bed...but he always stays in bed and goes to sleep on his own.
Best of luck! You can do it!
PS....if you are interested, check out Love and Logic (books, website, CDs, etc...) Very good parenting material - addresses common issues, such as bedtimes, leaving the house on time, giving kids choices, temper tantrums, etc...