Here's the calculation:

The oxidation number of K is +1.

The oxidation number of O is -2.

Let x be the oxidation number of Br.

In a neutral compound, the sum of the oxidation numbers of all atoms must be zero. So, we have:

1(+1) + 3(-2) + x = 0

Simplifying the equation:

+1 - 6 + x = 0

+x - 5 = 0

+x = +5

Therefore, the oxidation number of Br in KBrO3 is +5.