You can see that Scandium is in group 3 of the periodic table. This means that it has 3 valance electrons and will have an electronic configuration of [Ar] 3s2 3p1. In order to achieve a stable octet, you can see that removing 3 electrons will give it the electronic configuration of Ar, thus making Sc3+
Scandium will probably exist in the +2 and +1 state, they are just not very stable (If anyone definitively knows, I will be very interested). In the +1 state, it will have a stable filled 3s2 sub-shell and in the +2 state, it will have a stable(ish) half-filled 3s1 sub-shell. Scandium can also exist in higher oxidation states, but this cannot be achieved chemically as the energy needed to remove electrons from the stable octet would be too high.