To determine the oxidation number of Br in BrF3, we can use the general rule that the sum of the oxidation numbers of all atoms in a neutral compound must equal zero.

Let x be the oxidation number of Br. Since there are three F atoms with an oxidation number of -1 each, we can set up the equation:

x + 3(-1) = 0

Solving for x, we get:

x = +3

Therefore, the oxidation number of Br in BrF3 is +3.