regardless of the difference in their real life ABs (neither has more than 600 PAs), if they play full time when healthy (start every game and never get removed):
Over a full season, not counting post-season, an injury-2 player will miss about 189 innings (full games plus parts of games), and an injury-3 guy will miss about 270 innings.
The formula is complicated, but a good rule of thumb is this (applies only to players with fewer than 600 PAs):
Subtract the injury rating from 23. Multiply the result by half the injury rating. Multiply the result by 9 innings.
Example for an injury-3:
23-3 = 20. 20 times 3/2 = 30. 30 times 9 =270 innings. Again, that's if they start and finish every game when healthy. The numbers will be lower if you occasionally remove them from the line-up or remove them in the middle of games.
Important caveat: The higher the injury rating, the higher the standard deviation. So with a higher injury rating, you run a bigger risk of lots more innings missed if you're "unlucky" (i.e., you have a bad bell-curve year with the player, in terms of standard deviation). The formula above gives the AVERAGE over all seasons, but in any given season the standard deviation could kick in and give much better or much worse results.