I think it's proper to compare it to the Speed enhancement.
Flurry requires a full round action and gives you a -2 penalty to all your attacks.
Speed applies whenever you attack and doesn't give any penalty to attacks.
Speed is a +4 enhancement and is clearly superior to flurry.
On the other hand, existing +2 enhancements include:
Energy Burst, Thundering, Bane, Holy, Lawful, Unholy, Chaotic
I think that Flurry is superior to the existing +2 enhancements (especially because it could stack with them and a +1 Holy Flurry weapon would be a good deal better than a +1 Speed weapon)
Consequently, I think +3 is the right place for it.