a Beard is on your chin and up the side of your face. Moustache is between your nose and mouth, and sideburns grow from the edge of your hairline in front of your ears down (which would connect to the beard if someone has one). Whiskers is a term that people (at least in AE) use to describe short facial hair. So, if my husband decided to grow a beard, as it grew, it would start out as having whiskers, then it would get long enough to be called a beard. some also use "stubble" for the short growth of hair. Wiskers are also the long thick hairs that are on dogs and cats near their nose.
If someone has 1 sideburn (besides looking very funny) it would be a singular sideburn. But because most people would have one on each side, it's not usually used in the singular form.
Whiskers is the same way. most people have many short hairs that would as a whole be called "whiskers" but for an example, a woman might say "I have a lone whisker under my nose that i have to pluck" in this case, it was used int the singular form, because it was talking about only 1 hair.