R.,
My daughter, who will be three in December, recently started having problems going to sleep. She has always been my great sleeper, I'd put her in her bed and wouldn't hear a peep until morning.
This summer my daughter changed from going to sleep peacefully to going ballistic at night time. So, last month I talked to my ped. and she said it is just a phase. I remember struggling with my son at the same age. We started a routine and did everything we could to make his sleeping environment happy. We even started having our cat sleep in his room on the floor next to his bed. We told him the cat was a super hero and could protect him through the night. We did a nightly routine of bath, books, lullabies and then bed. We bought a CD player that is also a sound machine to create white noise.
We are doing all the same things with our daughter, but with limited success. I don't know that there is a secret trick, so you just may need patience.
By the way, our son occasionally still comes in our room at night when he has a bad dream and he is five.
Good luck