In English you'd usually use an "are". You three are against six (especially if this was the whole sentence). Although I'm assuming this might be used without an "are" solely in a sports or military environment where coaches and refs might cut down on words just for time). 3 February 2019 (I'm not touching the Hebrew, I'm learning too).
3 on 6 should be accepted. You would say this for example, in a game where there's teams playing against each other.
Why not you three 'are' against six? Assuming we need to say "are" in English.
See what I wrote above. You would normally use "are" unless you were in a situation involving sports or the military (or somewhere else where you were pressed for time, like a TV game show).