That is not always the case, but it is interesting to see for sure. Are there many other ducks in the canal, & r they all grouped in groups of 3? Or is this just one group of ducks in particular that seem to do this? Also, which species of duck r u seeing? Are u sure it is 2 males & 1 female of the same species, or is it 2 males with an immature male, or perhaps 2 different species? Sorry about all the questions. : )
I know it is kinda confusing, but it might help answer ur question. This time of year is past the breeding season now, & ducks r not mating this time of year. They do, however, group togther to feed & find food. Depending on where u live, once ponds & lakes freeze over, u will find large flocks of ducks & other birds on larger bodies of water that have portions of the water that does not freeze. Here they will feed & flock togther during the winter months. Once spring arrives, they will often go on their way to mate & raise chicks.
Ducks do hang out with each other, & form groups with ducks of the same species, or ducks that fall into a similar category. What I mean is this: Mallards & Black Ducks r different species, but they both fall into the category of 'dabbling ducks.' They feed on the surface of the water, & do not typically dive great distances to feed. These ducks often group togther. You will see small or large groups of them togther, feeding out on the water & on the shore. They're also commonly seen at duck ponds, & can become pretty tame when being fed by humans on a regular basis.
Other ducks, like Ring-necked Ducks & Scaup, r called 'Diving Ducks'. Where I live, we get European diving ducks called Tufted Ducks, & they too r diving ducks. These ducks do not dabble on the surface of the water; instead they dive down to the bottom & feed. They do not hang out with the Mallards or Black Ducks, they hang out with the other divers. You will often see here mixed flocks of Ring-Necked, Scaup & Tufted, both male & female, all diving & feeding togther. Sometimes u see 1 male, 2 females. Sometimes u see 6 males & 1 female. Or perhaps, just 1 male with a flock of females. It always varies.
Hopefully this helps! I do not personally think there is any reason in particular why u r seeing 2 males with a female. I just believe they're grouped togther to feed, & i'm sure if u watched them long enough, they'd group togther with other similar species as well. Hope this helps shed a little light on ur question! Take care.