a number of bus operators including easy has blocked off 6 seats in the 319 and have approval to run with 3 dollies just like a 149 seat 737.
Not "a number of bus operators", just easyJet! The difference is that easyJet ordered the A319 with TWO overwing exits on each side in order to fit an extra 6 seats (nothing like having a unique type!)
The A319 was originally certificated with 150 pax seats and 3 cabin crew. A second certification was carried out on an A319 with an extra pair of Type III exits, 156 pax seats and 4 cabin crew. Agreement was reached with the European Aviation Safety Agency (EASA) for this aircraft to hold two certification classes, provided it was modified in accordance with an EASA approved procedure and mandatory maintenance procedures were in place, this is a special case which has been addressed "differently" by EASA.