Hi S.,
You've probably already tried something like this, but when my daughter (also 3 1/2) has trouble sleeping in her own bed, I do something to make her bed seem "new" or "special." She hits a week-long spell every few months where she's convinced that she can't sleep in her own bed and she needs to come into mine. I've even just moved the bed to a different position in her room before and that satisfied her. If your bed seems comfier, maybe a comforter or new pillow that makes her bed seem comfy too? Perhaps she could sleep with a pillow or something from your bed. You could also make a compromise, she falls asleep in her bed but can come to yours in the morning? Having limited space makes it really hard to distinguish between her space and yours, I'm sure.
Good luck to you! It sounds like the biggest reason your daughter sleeps in your bed is that you two have a wonderfully close relationship and she feels safest with you, and that is something you should be proud of.