Hi there. I don't really use Fate Points for Combat Maneuvers in my campaign, but I can understand why you'd want to use them. I guess I'd ask: what would be the duration for allowing a Fate Point as you describe it? A combat round? The fight? One roll?
Also, if you're looking along these lines, another option might be allowing maximum damage IF the hit is successful (they should spend it before the attack roll, similar to the ability paladins get in DnD-Smite Evil). Actually, along this line of thinking you make the attack bonus equal to the STR modifier or CHA mod or REF mod instead, but to a minimum of +1 enhancement bonus.
Another option might be spending one fate point to allow a re-roll, or conversely adding a bonus of +1d6 to the roll SO LONG AS YOU HAVEN'T FLAT OUT TOLD THEM THE DIFFICULTY. (ala, they roll to hit, roll a 14 and look at you and you ask "Are you happy with that roll or do you want to spend a Fate Point to add a d6 to the total?" Of course, if they've been fighting for several rounds they'd already know the difficulty to hit, so it can be manipulated this way, but all rules are by eager players/mischevious GMs. :twisted: I just think +3 is pretty high as a flat base, but remember, whatever anyone says, it's your game-your group-your world. Whatever you decide, stay true to it and your players will have a fair time.
Hope this helps. By the way, these suggestions were posed by our group when we "playtested" the game before I started the group on it's first adventure, but since the basic game book doesn't provide for such options, and knowing my group, I decided not to use these.