Is this 100% polyster?

I believe it is 100% polyester.

I'm 5'3 and 110lb and 14 years old can i still fit this? If yes what size should i get?

Definitely. I'm approximately the same size as you so to answer your question I tried on my daughter's large (8-10) costume and it fits with room to spare.

My daughter where's a 3t, will a small fit her?

My granddaughter fit the small size perfectly on her 5th birthday and she is normal height and weight for her age (sorry I don't know exactly how tall she was - probably 40-42"). However, I had also bought a size medium and she actually liked to wear the bigger size because of the longer skirt. Her sister (one year younger) then wore the small size. Even though it might be a little bit big for your daughter, I think the small would work, especially since the neck runs a bit tight. With an elastic waist, the only issue would be a slightly longer skirt and she can play with it longer since there will be some growing room. Even not fitting perfectly it is super cute!