I'm fairly clear on the rules govening the multiplication of exponents with the same base, but what about adding them?

I.E. 2^5 + 2^5 + 3^6 + 3^6 + 3^6 = ?

The first part 2^5 + 2^5 is 2^6 because one is adding one 2^5 to another 2^5

but using this logic would 3^5 + 3^5 + 3^5 equal? 3^7?

The latter is incorrect, it should be 3^6? but why?