Is this an example of common knowledge?
This is not a common knowledge and should be best supported by some reference. Example reference for this could be market analysis papers, industry analysis reports or, at the very least, interviews with people from company X identifying A, B and C as competitors. The reason these claims should be supported is that you, personally, may not know some details about how A functions (making it target other audience than X) or miss a competitor D who is snapping at X's heels.

Now, how strict the requirement to support the claims is varying, depending on what kind of report you are writing. If it is a business plan for company X, apparently, you don't just use your personal observation, but should base your claims on something. If you are writing a general overview of some product that X, A, B and C produce, you might get away with using your personal observations, but it's still better to back up your claims with sources.