Oxidation number of Mg3B2?

The oxidation number of any Group II element in compound = +2
Mg is a Group II element.
Hence, oxidation number of Mg = +2

Mg₃B₂ is a neutral compound, a thus the sum of oxidation numbers = 0
(Oxidation number of Mg)×3 + (Oxidation number of B)×2 = 0
(+2)×3 + (Oxidation number of B)×2 = 0
(Oxidation number of B)×2 = -6
Oxidation number of B = -3

Oxidation number.....

We normally don't think of the oxidation number of a compound, although, if we did, it would be zero, since the sum of the oxidation states of the elements must add up to zero for a neutral compound. Instead we deal with the oxidation states assigned to each element in a compound.

Oxidation states of Mg and B in Mg3B2
Mg .... +2
B ....... -3
+2 +2 +2 -3 -3 = 0
Mg Mg Mg B B

There's only one catch. You won't find any Mg3B2, especially in the literature or on the stock-room shelf. A compound of magnesium and boron has the formula MgB2, and is usually called magnesium diboride.

In this compound, MgB2, which actually is found in the Handbook of Chemistry and Physics (while Mg3B2 is not), magnesium has an oxidation state of +2 and each boron has an oxidation state of -1.

Wikipedia lists the possible oxidation states of boron as: 3, 2, 1, −1, −5
Notice there is no -3.