I thought I'd share what worked for us, for another perspective and option for you in case the other methods don't work for you. My 3-year old boy started sleeping in a twin bed at 24 months, a month before his new brother was born so I wanted to free up the crib. He slept great in his crib also, so I wasn't sure how the transition would go. We'd talked about the big boy bed and tried to make it an exciting experience, but when I finally decided it was time to make the switch we had to say "no going back" or it would confuse him! So, the first night was SO HARD. He SCREAMED louder than I'd ever heard him, and this was after a nice calm bedtime routine, story, etc. As soon as I would leave, he would run out of the room. At first I thought I'd try to do the Super Nanny method of just putting him back and eventually he'd stop running out, but after 8 times in a row with no rest it was just making me angry and I knew that wasn't a good thing. Then, I put up a pressure gate, but he just knocked that right over (and fell on top of it, crying..). So it was time for plan C. I closed the door and held it closed (someone else I knew reversed the doorknob so you could lock it from the outside- just another option), and waited out the screams. It was so hard, but it was seriously just 4-6 minutes of him trying to pull the door open and screaming/crying (I timed it) and then he calmed down, and climbed back into bed. He whimpered a few more minutes, but an hour or so later I checked on him and he'd climbed into bed, laid on his pillow, pulled up the sheet, and was fast asleep! He'd also turned on his small bedside lamp, and that probably helped him. To this day he sleeps with the closet light on and a nightlight, something he never did in a crib. So maybe there just needs to be some changes, and things to keep her comforted at first, but you have to be consistent.
Also, that next morning he woke up really really early (like 5 am) and I said, "No, it's not time to get up yet" (he was super whiney too) and put him back in bed and did the door holding thing again for just a couple minutes, and he went back to sleep. They need to see that bedtime/naptime means staying in bed, or the consequence is closing the door to keep them in there. It sounds mean to do this, but isn't it meaner to deprive them of clear expectations and consequences, and a good nap and good nights' sleep? Think of the long term- a short time of crying and testing you will produce days, weeks, and hopefully years of good sleep for you all!
This may not work for everyone, but it sure worked for us. My son sleeps 11-12 hours at night these days, and sometimes takes a 1-2 hour nap (he'll be 4 this summer).
I wish you the best. Tough love! :)