It’s not black and white kind of decision though.
KV3 is a unique tank, it has 3 viable top guns, not 1 like most of the others.
The 122 is similar to the 152 on KV. Damage is nice but don’t even bother shooting far targets with it. Inaccurate slow reloading gun on a slow tank can result in a lot of frustration. The 122 has the least DPS already and every missed (bounced, crit with no damage, tracks, etc.) shot affects it drastically.
If you prefered the 152 on KV, you will like this one as well. The big difference from the 152 (at least the pre-HE nerf 152, don’t know how it plays now) is that you don’t care where the 152 HE sheel hits the enemy. With the 122, you do and accuracy is a problem.
And don’t kid yourself, 175 mm pen (especially with this kind of accuracy) is not nearly enough to feel comfortable against tier 8 heavies, if that’s your goal. And mediums will tear you a new one with your slow reload.
The 107, on the other hand, loses 8 mm in penetration and about 20% in damage but it reloads much faster and has a bit better accuracy. Penetration-wise it’s pretty much as reliable as the 122 against tier 7 targets (and about as useless against tier 8 :)).
The 100 has pen of the 122m, is more than twice faster, is even more accurate than 107, has much better aim time but loses about 40% damage comparing to 122.
In the end it all depends (as usual) on your playstyle.
The 100 is much better against mediums or when flanking or when “sniping” (sniping is always in quotes when talking about Soviet tanks) or when playing aggressively but it loses its advantage in “around the corner pew pew” kind of fights.
The 122 is opposite to that - best at “around the corner pew pew” at mid distance (not accurate to shoot far, too slow in close quarters).
The 107 is in between.
Personally, I felt that KV3 is too slow to play as aggressively as the 100 requires and I didn’t enjoy getting smoked by mediums when driving with the 122. So after trying all 3, I settled with the good old 107.
OTOH, might as well get used to the 122, since that’s what you will use on IS and some part of IS-3 (although it reloads faster on those two tanks).
KV-1S, though, would be perfect for the 100 but it doesn’t get it. Go figure.