UA does not allow stopovers within a single region. You are allowed one openjaw. Between multiple regions, you are allowed a stopover OR an openjaw. With UA miles, you could do YYC-MIA, MIA-BOS or YYC-FLL, FLL-BOS. You might be able to do BOS-YYZ and MIA/FLL-BOS, but you might run into rules about the length of the open-jaw segment.

I don't know about AC's rules.

EDIT: Actually, I was wrong. YYC-MIA, MIA-BOS (or FLL) is not valid.
YYC (51°06'50"N 114°01'13"W) MIA (25°47'36"N 80°17'26"W) 121° (SE) 2490 mi
MIA (25°47'36"N 80°17'26"W) BOS (42°21'47"N 71°00'23"W) 22° (N) 1258 mi
BOS (42°21'47"N 71°00'23"W) YYC (51°06'50"N 114°01'13"W) 301° (NW) 2097 mi

BOS-YYZ, MIA-BOS (or FLL) is also not valid.
BOS (42°21'47"N 71°00'23"W) YYZ (43°40'38"N 79°37'50"W) 284° (W) 446 mi
MIA (25°47'36"N 80°17'26"W) BOS (42°21'47"N 71°00'23"W) 22° (N) 1258 mi
YYZ (43°40'38"N 79°37'50"W) MIA (25°47'36"N 80°17'26"W) 181° (S) 1233 mi

What would be valid for an open-jaw on UA would be YYZ-YYC, YYC-MIA(FLL).
YYZ (43°40'38"N 79°37'50"W) YYC (51°06'50"N 114°01'13"W) 300° (NW) 1675 mi
YYC (51°06'50"N 114°01'13"W) MIA (25°47'36"N 80°17'26"W) 121° (SE) 2490 mi
MIA (25°47'36"N 80°17'26"W) YYZ (43°40'38"N 79°37'50"W) 1° (N) 1233 mi