Me and my friend like to play the card game yugioh. We always fight over this one rule. I have the legal version of obelisk the tormentor when I have my 3 monsters on the field and say I'm tributing these monsters for obelisk can my friend activate compulsory cannon on one of my sacrifices. One of obelisks affects is you can't negate the summon so can my friend activate this card or no?

No, Obelisk also has an ability that states that when he is normal summoned (which you are doing) monster, spell and trap cards cannot be activated.
I'm not familiar with the card compulsory cannon, but unless it is in the card description, you cannot chain to a tribute, only the summon or effect you are tributing for.