On the box near the profession name.
If you have 3 slot, you can't do more than 3 profession.
So, if you want to do 3, put 1 on each profession and 0 to the others,
If you want to do 2 profession, put 2 in one, 1 in an other and 0 for the rest,
If you want to do only one profession, put 3 in it and 0 on the others.