Did your actual total rating actually go up by +3, or does it only say +3 on your history?
A user's impact on your rating is effected by their existing rating, as far as I know. However, when they rate you, it always shows up as +3 on your rating history (due to a bug) and as +3 on the green pop-ups.
If I rate you up, you get +4, but it'll show as only +3. (+4 is the highest, as far as I can tell). I mentioned in another thread that when a user with zero rating rates me up, I get +0. Although, oddly, in that case, it actually shows up as +0 instead of +3.
I chose a random user who I know makes friendly and helpful posts:
I uprated that random user with +4 points:
But it shows up as +3 in his rating history:
(pardon the blatant and uncalled for flattery, I'm just trying to be humorous )