1. Why can't you have 3 electrons in one orbital?

2. Why does Indium break the left to right trend for ionization energy?

1. The best answer I can give is either of the following"

a. It's the nature of th beast.
b. The mathematical solution of the Shroedinger equation gives two values for ms of +1/2 (h/2*pi) and -1/2(h/2*pi).

2. In has added the first p electron which makes it further out from the nucleus and that makes it easier to pick it off. Therefore, the ionization potential is lower than that of Cd (a full s orbital).