First you need to determine exactly what zone you do live in. You can view the Ontario hardiness zone map on this website. Some parts of southern Ontario are far warmer than zone 3, below Windsor the map shows zone 7b! If you up near Thunder Bay though, you would be correct in assuming it is zone 3.

Growing Morning Glories from seed that far north, you should start them indoors about a month before it is warm enough to plant them outdoors. Your season is very short and Morning Glory needs to get large enough to flower before August if you are going to enjoy color on your fence. Due to the shortness of your season I would plant one every 3 feet. They shouldn't take over much space at the base, and if they do start sticking out in the way as they grow and fill, tuck the errant stems up into the plant and it will climb with the rest of the plant. Netting everywhere would be unsightly for weeks. The plant can climb a single strand of plastic line. Heavy gauge fishing line works great. Just anchor it in the ground behind the base of the plant by tying it securely around a ground stake and anchoring it to the top of the fence board above with a staple or nail that is also well knotted to keep the strand taut and in place under the weight of the future vine. Once it reaches the top of the fence the vine will spread along the top and spill over.

Make sure to give them a good fertilizer so they grow and fill as fast as possible. Mix up those colors and enjoy the show. They are such beautiful flowers!
Tammy Clayton