Merging them has a random chance of producing Magic Coins - Gold, Dragon Egg Chest/Dragon Nest Vault (Midas Ducks variant) costing 17 and 85 gems respectively to open, and Golden Apples of the next level.
Price wise, it is recommended to sell the level 1 apples. If you wait and merge Level 3 apples you get 1 nest chest and 2 egg chests, equal total of 80 coins, while 3 level 3 apples are worth 90 coins. However, 3 Level 2 apples give 45 gold (much better than one L3 apple), and 3 L1 apples is 24 gold, again much better than a Level 2 apple.
The recommendation above is true if you don't have space in camp. If you do, merge always 5 of L1 and L2, and sell only L3 and the boxes you get along the way. The real cash would come from the Gold Coins you would accumulate and merge as well, as merging Gold Coins into higher tiers will create even more coins.
|1||1x1||Epic||Golden Apple||Fruit from King Midas's Garden.||8|
|2||1x1||Epic||Giant Golden Apple||Touched by a Midas Duck.||15|
|3||2x2||Legendary||Golden Apple of the Cosmos||This fruit has a strange, goldeny-magic about it.||30|