Combine the ricotta cheese, egg, Parmesan, and 2 cups Mozzarella cheese in a medium bowl.
Place cheese mixture into a piping bag or Ziploc with corner cut off.
Fill each pasta shell with cheese mixture.
Place stuffed pasta shells (uncooked but filled at this point) on top of the meat layer.
Carefully pour water on top of pasta shells.
Pour pasta sauce evenly on top of pasta shells.
Place lid securely on top of Instant Pot and ensure valve is in sealed position.
Cook manual, high pressure for 8 minutes followed by a quick release.
Once pin has dropped, remove lid and sprinkle remaining 1 cup of mozzarella cheese evenly on top of pasta shells.
Place lid back on the Instant Pot and allow to rest for about 2-3 minutes, or until cheese is melted.
To serve, scoop shells onto plate. Make sure you scoop to the bottom to get some of the meat into your portion!