That's a weatherhead (as already mentioned in a comment) and you don't generally "remove the whole thing" (it should be glued on) you remove the top of it - in your case, apparently by removing the two screws at the sides, then lifting it off (mine snap off and on, to the best of my recall.)
If it has your "internet copper" in it, there should be no line voltage power in it, so verifying what the third cable actually is, is going to be required, and correcting it (removing it from this conduit) if it is line voltage power would be required before doing anything else with it.
The knockouts are there to run cables or wires through, so you can open one up to add a cable, once you've eliminated the possibility that there is line voltage on one of the cables. That might involve a hammer, a hammer and a punch, a utility knife, or a drill. If cutting or drilling, make sure you don't damage cables already in place.
If it's hard to reach even from a ladder, you need a bigger ladder, or you should rent a manlift, or staging, or some other means to safely access it. Emergency room visits and funerals are both expensive.
