New Bed for 3 Year Old

At three he totally should be ready for a big boy bed and a twin is the way to go since they outgrow toddler beds so fast. Get a bed rail and put it on there so it feels secure for him and he doesn't fall out. Don't wait until he crawls out of the crib and he needs to gain the much needed independence of being out of your bed. It will do wonders for your marriage and him feeling self assured.
Have him pick out his own sheets, you can get a cool bed in a bag with a cool character for like $30 to $40 with everything at Walmart or Target. Just be patient but make it understood he has to be a big boy and be in his own bed, really, helping him sleep by himself in his own bed benefits the whole family!

I totally agree with the previous posters. However, if you are already having issues with him going to sleep on his own you will need to work on that too. Try reading "The No Cry Sleep Solution" - most of it is geared to babies (which will give you some good tips for your daughter) but there are some great ideas in there that work well for older ones in establishing good sleep habits. I hope it all goes smoothly for you!

J.-
We just recently bought our 4 y/o son a new bed, and he didn't really have much issue with staying in his own bed, he just hated to go to bed, and it has really helped. Let your son help you in rearranging his room, buying sheets and blankets for his new bed, and maybe let him pick out a new lamp or some new posters or something for his room, so he feels more like it's his decision and it's more his room.