My 3 1/2 DD still does this - every night. I've tried putting her back to bed - over and over and over. I've tried sticker charts, I've tried bribes of "bigger" toys, a new bedroom set (she has a crib/toddler bed) I've tried "All your friends sleep in their rooms!".... Nothing works for her!
I think that part of it is that she's an only child, and she just doesn't want to be alone. She will flat out say - I just miss you - or - I don't want to be in my room all alone. I mean - how do you explain that to her? While you and your hubby are all nice and snuggled in your room together?? (Oh yes - we even bought a LIFE SIZED Elmo for her to sleep with!)
I also think that at 3 - 3 1/2 - 4 - they are going through a "where's Mommy" stage, and mine is constantly asking - where are you??? so - at night - in the dark, and quiet - it's even more of an issue.
I know that it's hard - but congrats to those that can reassure their little ones, and get them back to their own beds - a time or two! :-)
There are others of us that are still sleeping with feet in our back, and elbows in our kidneys. But I, for one, am just blessed to have that little foot in my life - even if it's in my sleep deprived face some nights!
Hang in there - we are all feeling your pain!