Support for Tile floor
I'm not sure if laminating a sister to each beam would count as three inches (I don't see why not if done properly, for at least as far as deflection is concerned; load calculation I have no idea), then it would be 3 x 7.75, and according to your link (L/673) that's OK for tile.

12 feet of 2x8 is pushing it, for sure. Sister each beam and install blocking.

My answer here, provides what insight I have for laminating beams; e.g., nailing schedules.