I let mine come to my room and sleep on a sleeping bag on the floor at the foot of the bed for a while. Once DH had gotten up and gone to work, I didn't mind a kid in my bed, but the rule was that the space had to be empty.
Three in the bed just wasn't practical - I don't sleep so well that way, and DH is a big guy and takes up a fair portion of the bed.
To be fair, the sleeping bag solution did become a problem when two girls wanted the room and our bedroom is too tiny for more than one sleeping bag, but it was a nice temporary solution. And it ended for good only after I threw a HUGE strop....when Natalie was 5YO.
Can you maybe go around the room with her and do something like, say, spray lightly-scented water - water in a spray bottle with a drop or two of lavender essential oil? - around the room to help it be a more soothing place? Can you re-arrange her room a bit, or hang a special picture on her wall?
Melatonin has also helped us break cycles like this. We only gave Nat mini-doses when needed, but it bought us some time and helped us get out of the room with her sound asleep.
But if she's truly that scared, it doesn't sound like this is the time to enforce the rule and force her to stay in her room alone either..... will have to give this more thought.