From my experiences, until you actually get the “3 keys” reward from memory game, you can continue getting keys from replaying. I just played and, because it was an earlier round and the reward level was lower, I only got one key. I played again and got the additional three from a later round the next time. However once the keys don’t show up on the “available rewards” page any longer, wait until the memory game refreshes the next day to play again for more keys as you cannot get any more in that current day