But you could fly the 12k, then two A3 flights would take you to silver, and the next four A3 flights would count towards Gold (along with 24k miles). So you can still fly all the 6 A3 segments in one go (although best to make sure that flights 2 and 3 are on different calendar days), so long as you have done 12k miles before or including the first 2 A3 flights, on one trip to/around Greece.

So instead of your suggested 6 flights, 12k miles, 24k miles, it would be 12k miles, 6 flights (split between #2 and #3), then 24k miles to get to Silver and then Gold.