iterate over vs iterate
The meetings may happen, occur, come up, or are scheduled as often as you please.
An iteration is more than a repeat—it is one round of cycle. You could call one pushup in a set of 15 an iteration.
While #2 is worse (meetings get iterated over every three weeks), the meetings in #1 may occur every three weeks, but with different content, not the very same content. So they don't iterate. Only in the film Ground Hog's Day would they iterate. Again.