b+3 is one of Dragunov's two homing attacks, the other being f,f+3. b+3 is the fastest homing move in the game at i14 and has good, almost deceptive, range. Its major faults are that even though b+3 comes out in 14 frames, it's a high and has no special properties on CH, as well as the same frame advantage as hit and CH (+3).
Once you get past the -9 on block and +3 on hit/CH, b+3 is still a fairly strong option and it's worth throwing out every once in a while. Using b+3 as a poke, it's a great alternate to d/f+4 which is plagued by tracking issues and mediocre properties on hit. A novice mistake while using b+3 is to use the attack when the opponent is very close to you, opposed to using the attack at near max range or at max range. It seems that the closer the opponent is to you, the less effective the attack becomes. This is caused by b+3 being fairly poor on whiff, now granted you're not very likely to whiff with b+3 (especially once you get a feel for its true range) but it's hit hard by random ducking as well and punch checking at close range.
I find that being unpredictable with b+3 poking is an extremely strong strategy, even more so once you're comfortable with iWR 2 pressure stringing. The animation is very misleading and very abrupt, namely due to the i14 speed, and is near invisible. As I said in the section before, learn the range of b+3 and I'm not just talking about when you should use it as a punisher (e.g. Bryan 1+2,2), but when risk versus reward will stop them dead in their tracks with a well placed b+3.
Theoretically the only risk that you're taking with a max range b+3 is against generic d/f+2 style launchers, but at max range you'll trade and their follow-up combo should be extremely hard to perform, if not impossible. The same goes for max range ducking, I haven't done any real testing regarding this, but realistically only one or two characters might have the range to duck to WS launch at max range but I'll have to do testing to be sure.
UPDATE: It seems that only characters with a WGF punisher, namely players who are able to perform DWGF/DEWGF are a major threat to max range b+3.