As long as you can physically count the individual hairs, then you can use the word "hairs" as a plural noun. However, if you are talking about a picture you are looking at, then you would say either "The monster has only three hairs" or "In the picture, there is a monster which has only three hairs". If you say "A monster has only three hairs" then you are making a general statement making it sounds as if all monsters have only three hairs.
Note the correct spelling of "separate".
Interested in Language