Just as a basis:
Normal 2 has a motion value of 12
Normal 3 has a motion value of 10, but has the possibility to ricochet (up to 4*10 motion value)
Most gunners only use normal 2 for numerous reasons. 1. More damage (higher motion value) 2. More ammo per clip (for most bowguns) and 3. More reliable damage on the monster weakspot (They dont care about the possibility of a ricochet on a less weak body part). All these add up to more calculated and consistent clear times as there are no random components with your shot
And then theres Normal 3, often shadowed by the former. The main reason why many dont use this shot is because it is very diffficult to hit the right part of the body to ensure the ricochet. Also, many bowgunners only shoot the head, which almost never results in a ricochet (since there is no crevice for the shot to enter the body and do multiple hits). Normal 3 is, however, very good versus something like Dahren Mohran, as you can stand directly against his crest or blowhole and guarantee yourself all ricochets land (40 motion value PER shot, holy). Also in third gen Normal 3 had a higher chance to ricochet underwater, making them extremely good in some situations.
Now for my personal opinion. Since Normal 3 has a slightly higher critical range over Normal 2, I tend to use them for fast monsters that move a lot online so I can fire from slightly further away and still hit critical (if I have evade extender I use Normal 2 though).
I find Normal 3 more fun for a less serious hunt, as I actually like the random component of the shot to do huge damage. Something else to keep in mind is that if the first hit of the Normal 3 crits, then all ricochets will also crit, resulting in a flurry of bright flashes, multiple part breaks, and sometimes a stagger.
In the end, use what you believe to be the surperior shot type. All pros will tell you Normal 2 no doubt. I use either depending on the situation. Im sure there are a couple members of the Normal 3 Master Race lurking as well.
In the end, both are perfectly viable and dont let anyone tell you otherwise. Hope this helps.