Since there are only .3 seconds left, that means a team can only score by tapping the ball into the goal... they can't catch and shoot it. So I understand allowing the game/quarter to end, even though the horn sounded before anybody on the court even touched the ball.

However, going strictly by the rule book, we should treat this as an interruption and re-do the thrown-in.

One big piece of information is missing... did Team A stop playing when the horn sounded, when they had a chance? If they did, then we have to re-do the throw-in. If they didn't, then go ahead and end the game/quarter.