How do you balance Co(s)+Cl2(g)?
I keep getting this incorrect: 2Co(s)+3Cl2(g)-> 2CoCl3

Explanation:

Cobalt either has two or 4 as it's valance number == usually 2

Co + Cl2 ==) CoCl2 and that balances it.

I was quite surprised to find out that that Cl3+ actually does exist, but I'm pretty sure only in a grade level about yours. It sort of looks like this

Co3+  + 1e- ===> Co2+

If there was a Co3+ ion, your equation would be correct the way it is done. That's why I went to check it.

you need to have a picture of the atom that way we can answer it