I suspect there was an error - that's probably not what they meant. Nonetheless, there's no problem. It simply means that out of 500,000 sent out, that 499,970 would win gift cards. 3 win "grand" prizes, leaving 27 schmucks who didn't win a thing.
But, 500,000 seems like an awful lot of them for a dealership to send out. I suspect it was closer to 50,000. I would think that they would have some sort of mailing list such that there were no more than one per household, and that they used some sort of criteria to focus their sales. E.g., using historical data to target neighborhoods where more people had purchased new vehicles from dealers. That is, and I don't mean this offensively, if they targeted some neighborhoods, they'd have thousands of people standing in line who all were willing to wait 1 hour for their free $5 gift card, while of those 1000's of people, they'd be lucky to sell one new vehicle. Given the cost of postage in such a case, they'd lose money doing this as a promo. Hence, 500,000 seems awfully high to me.
Then again, perhaps the odds really are 1/500,000, and they only printed out 5000 of them. If that were the case, and they were truly printed at random, there's only about a 1/10,000th of a percent chance that all three winners were printed. And, barely more than a 1% chance that any grand prize winners were printed. Probably depends on state laws regarding such promotions though.
Usually these things have a requirement that you have to scratch it off in front of a representative or that they have to.
I get at least 1 of those a week; I don't recall any that have such a requirement in years. What they do is, "get three matching squares in a row, and you win!" You scratch them off, and all 3 are a kia, or all 3 are a picture of $2500, or all 3 are an atv." The rules don't say you win what you got 3 of... only that if the three match, you win. So, it suckers people into going to the dealership to find out that their 3 matching atv pictures means they won a gift card. (Else, I'm the luckiest and dumbest person on the planet, because I always get 3 matching pictures, and have never wasted my time.)