There's a setting via dipswitch on the board to make it give you 3 free games if you get the high score, so maybe you're getting better with each game and always beating your previous score.

There's also 3 different free game score settings, so if those are low enough you would get several a game too.

Now if you're saying, you get the free game score (lets say it's set at 150,000) and instead of it giving you 1 knock it does 3 knocks, it might be a bad transistor although I don't think I've ever seen one fail like that.

Play the game, and when it knocks three times see if it gives you three credits, or it gives you 1 credit and knocks three times. If it gives you 3 credits, then it's software related and if it only gives you 1 credit it's something mechanical or electrical.