As a fraction, the exact answer is 5/6
In decimal form, the approximate answer is 0.8333
To get this answer, note how there are 5 ways to roll something that isn't a three (1,2,4,5,6) out of 6 ways total (1,2,3,4,5,6)
So you simply divide the two values to get 5/6 = 0.8333