I watched a lecture on your prayers series (3) concentrating during prayers.
Did I get this right?
Allah will forgive your mind wandering up to 3 times during salat, but if it does so more than 3 times, it won't be accepted and you should start over..
Or did I get it wrong?

No it doesn’t mean that the prayer is invalid and must be repeated. It means that the reward of the prayer will be significantly reduced, and Allah may not accept the prayer in the sense that it won’t have much value. Otherwise the prayer is still technically valid and does not have to be repeated.