I colonized provinces 3 and 7. 3 is the mountains and forests and the other is 7. Well it sounds like to me that it's best to build higher levels of fortress in the mountains and forests. Is it possible to box province 3 because it is at the border line of colonization?
nope, you can't box prov 3 ... you absolutely need 5 provinces in a "+" shape, and the center being the boxed province.
which means you were planning to box your plain province (8) .... classic ...
I don't know your map, but you should take a second look before colonizing again .... in the meantime, your province in mountain is by far your best location for defense.