Introduction:
Last week I was playing a novice Sergei player who would mix in b+2,1,3 with his normal attacks. While this strategy may be valid against completely new players, there is realistically no chance of landing this string on a decent player with even limited Dragunov experience.
There's three major problems with the b+2,1,3 string:
While the final attack can be canceled into a crouch grab, the speed at which it comes out varies from the attack, therefore it's not a true 50/50. Therefore it's possible to physically block the 3 and WS the grab.
This leads into my second point, since the speed varies, there's no reason to actually duck the last hit and possibly deal with the 50/50 mix up. While you're missing out on a WS launch combo, there's 0 risk involved to low parry and it is still possible to full launch the grab when it whiffs.
Characters with hop kicks, or any type of low crush attack, can simply jump over the 50/50 and full launch the string. The frames on b+2,1 (on block) are simply not strong enough for the 3 to interrupt them before they leave the ground.
So when should you use b+2,1,3?
There's only two possible cases where b+2,1,3 is viable. During wall splat, even though 3,1,2 is a more consistent option, and when the opponent is caught BT to you granting a small combo on hit. If what you're doing doesn't fall under one of these conditions, then I recommend not using b+2,1,3.