I'm terrible!

I might be wrong but I got 1/18

There are 36 outcomes all together and only 2 that have a 3 and sum of 7 so that would be 2/36 and simplify that which gives you 1/18

There are 6 outcomes where the sum  equals 7

Two of these  include one 3 ⇒   (3,4)  and (4,3)

So  the probability is   2/6   =   1/3