I'm way out of my depth with regard to how things will work in Canada, but if it follows the pattern used with USA leases the tax credit is added to the residual, thus reducing the "capitalized cost" and the monthly payments. [They don't have to do it this way, a better approach from the customer point of view would be to add the tax credit to the down payment, which would lower the capitalized cost and also lower the buyout price.]
What the residual value will be is an interesting question. For the Model S it tends to be around 51%, or a bit higher with lower mileage (kilometerage? is there such a word?) three year leases. Will the Model 3 get the same residual value as the higher priced S? I'm guessing that it will be a bit lower. But, assume 50% for three years @15,000 miles (24,000 km) per year. The numbers might look like this:
Cost = C$60,000
Residual = $30,000
Down payment = $6000
Tax credit = $14,000 (yes, the 3 will have five seats)
So, $60,000 - $6000 - ($30,000 + $14,000) = $56,000 - $44,000 = $12,000
That $12,000 capitalized cost, thanks to the generous tax credit, would make for a mighty attractive lease. I haven't the faintest idea of what lease interest rates are like in Canada but if it was at the current USA rate of 4.08%, the payments would be around $355/month, not including some small lease-only fees.
FWIW. These are just guesses on my part, based on incomplete information.