The US looks at your class standing ( position in your class relative to all other students) first and score on the NBME exam second( very difficult exam as you are tested on all the med school material) to determine who gets interviews.
So one course over three years is not that significant but you want to finish in the top 5% of your class over the first three years to be competitive.
For me the big work load was studying for the NBME exam while taking classes. You need to start working on that in second year. You will be disadvantaged as U of T DDS students don't take med classes like U of A, McGill and many US schools do. Get a hold of notes and texts from a med student and start studying if you are serious about specializing.