"In addition to" can add a sentence after it;
besides is a preposition and adv.It can be used as a preposition,followed by noun.
As well as is a phrase,followed by noun.
1.In addition to what I just described above for you,you can look at other materials.
2.Besides,I'd like you to know that I will be late for next meeting.
3.She is very good at the spelling as well as Math.
Bo