You will most likely gain seniority faster on the 757 and A300. The 757 because people bid out of it to the 777 or 767 for the WB pay.
The A300 because most people on property don’t bid into the FO seat from another plane and some A300 FO’s bid out to the 777 or 767 FO or 757 captain.
The Airbus hasn’t seen newhire in a long time (like the MD), that’s why the most junior pilot on it is senior to the junior 777 pilot.
At least that has been my experience.