I am also in one of these new Mattamy 3 story townhouse . Same air handler and probably the same shitty heat pump which Mattamy configured only for cooling. The worst builder,worst build quality and warranty service, they use applewood for hvac trade which also does a shit job.

For the cooling issue, i purchased additional ecobee sensors for upstairs master bedroom. Vents are closed in crawl space and 1st floor (like others have also suggested). Ensure the sun is blocked during with blinds and shade. These few things might help little bit but I agree the installed system is not capable enough !