We like the 2 tenants and they've lived in the triplex for several years, but they're paying under market rents and won't be able to afford a significant rent increase.

Even if you can afford it, is it the best thing for you to do?

Their mortgage is too high for them to afford but with my income/credit, I'd be able to purchase their property.

This is trickier because you don't want to wait until they can't afford the property.

If you’d be heart broken if you lost the property, make sure you don’t take out a bigger loan than you can afford to pay with zero rent coming in.

Discusshttps://wtop.com/dc/2020/01/dc-lawmaker-hopes-bill-will-protect-affordable-housing-and-increase-tenant-protections/

Section 8 adds a standardized lease, annual inspection and a subsidy to enable the prospect to afford your rental.You and Tenant sign your lease and the S8 lease. 

As a tenant it is beyond frustrating.

Or more, beacuse it’s more affordable then buying a home.  

Even with "rent stabilization", rents are still increasing faster than wages in these markets; ergo, the voters will eventually require their lap dogs....err, I mean representatives to pass new laws to make housing "affordable".