Do we say "three's a company" or "three's company"?

I would say that "three's a company" is more appropriate as an idiomatic expression. It is a short form of saying "three is a company". 

According to this website, the original idiom is "two's company, three's a crowd".

Then again' "three's company" may be correct too, since in the original idiom, it says "two's company", meaning "two is company".

So, "three's company" can be read as "three is company".

I think both are correct. However, to be closer to the original idiom, maybe we can stick to "three's company".