Size 2 and Size 3 will work. I tried the size 2 but didn't like the way it fit so I swapped it for a size 3. In my opinion it fits a bit better as it's built for the LCP/P3AT. It's a compromise holster as Blackhawk does not have a size specific for this pistol.
I'm glad I could comment on this because I was trying to find the answer to this question on my Iphone in the store when deciding between the size 2 or 3. Google came back with nothing. Of course, I bought the wrong one (size 2) and had to return it for the size 3. Not a big deal except the the store is over 40 minutes away from here.