One big difference between TB2 and TB3 is greater open time, aka the amount of time you have to mess around and align things, spread glue, etc. before you put the two things to glue together. One woodworker friend of mine said it was 10 min extra time for TB3 as compared to TB2. I'm not sure if that is the right length of time, but any extra time can be worth the extra cost of a bottle. Remember that atmospheric conditions can shorten the open time considerably. So doing a complicated glue up in hot, dry weather could result in a bad glue up with TB2 and with TB3's extra open time might turn out okay. This is really an extreme example. If you really need more time, then move to epoxy.
Back to Titebond, TB3 is supposed to be also better for gluing oily and other wise hard to glue woods such as osage orange or exotic woods like teak. So again this is a good reason to spend the extra money.
As for TB3 being waterproof, it's good insurance, but most bows are not subjected to the extreme that would make even TB2's weather-resistance start to be an issue. At the shop where I work, we make cutting boards glued up from scraps of hardwood lumber, we use TB1 which is not water proof, resistant, or any thing. These cutting board last for years of being rinsed and dried, it's the cycles that break down the glue bond. The ends do start to separate, but the rest of the length of the glue joint holds. The end-grain takes on more water anyway.
Most of us seal our bows with some finish, so water has to get through that layer first before it can get to the glue. If someone glues on a cloth backing with TB3 for example, then the glue itself could count as the finish. The best way to get around the extra cost is to borrow some TB3 from your neighbor or brother-in-law or someone else that you know. Tell them that you just need a little bit...
