I know that I'm often the odd man out on this, but I never really got into the petty fan club. I just feel like a parer is always better for air-work and for things too big for the parer (smaller on the board tasks) I'd rather use something with better knuckle clearance than most petty's like my Carter 225mm funa.
The only sub 200mm knives that I ever use that aren't a parer are my Carter SFGZ riveted funa because it's very tall for it's size and my 180mm Mizuno deba.
I like the suggestion to replace the petty with a honsuki as it's good to have at least 1 stouter-bladed knife. Plus, a good one could possibly replace both a petty and a deba if you're doing more poultry than fish.
I also have a fondness for big, 270 gyutos, but after using my medium-thick 225 Carter Funa I realize that the only time that I really NEED a longer blade, the need could be easily met by a 270 suji.
So, my vote would be:
- Small parer (<or= 90mm)
- honsuki (maybe a slightly lager, stouter one at ~180mm)
- 225 gyuto
- 270 or > suji
If I really HAD to get the set down to 3, my first choice would be to drop the parer, my 2nd would be to replace the gyuto and suji with a long, low 270mm gyuto (Marko or Masamoto KS style).