Originally Posted by IluvSQ

LGA-YUL-LAS-YUL-LGA is not a permissible routing ( cabotage) if all on one ticket on AC.
So you would have to do LGA-YUL-LGA on one, and YUL-LAS-YUL on another.

Furthermore, LGA is one of the most delay-ridden cities, I would be careful of
connections when leaving from LGA, and leave more time than from most anywhere
else.