A protein domain is a conserved part of a given proteinsequence and (tertiary) structure that can evolve, function, and exist independently of the rest of theprotein chain. Each domain forms a compact three-dimensional structure and often can be independently stable and folded
so your protein can include one , 2 , or 3 domains
Hi Amala,
You need to read followings articles to understand it better.
http://www.mrc-lmb.cam.ac.uk/tcb/pdf/chc/160_cosb_14_208_04.pdf
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2143930/pdf/9521098.pdf
and this https://en.wikipedia.org/wiki/Protein_domain
Cheers