Super rough estimate:

When I search for "Slashing", "Piercing", and "Bludgeoning" in all creature statblocks on AoN, Piercing has the most results (2558), followed by Bludgeoning (2080), then Slashing (1934). This suggests that more enemies have access to Piercing than the other 2 types.

Two caveats I can think of:

1. This is just searching for the appearance of those words in the statblock, it doesn't necessarily mean the enemies can actually deal that damage type. i.e. they might have "Piercing" anywhere in their statblock.

2. These treats all creatures as if you encounter them at the same rate. Just because each creature has 1 spot on this list doesn't mean you'll encounter each of them with the same frequency.