Ok, I’ve noticed over 60% of players had some difficulties solving level3. And here is an ultimate hint to the riddle.
As most of you already figured out that the missing ones are “F” “O” and “U”, the only thing left to be solved is how in the hell that “FOU” can be flying?
Apparently it doesn’t, but wait a min, what if I re-arrange the order of the letters? what if it’s not “FOU” but “OFU” or … …
There you go, you’ll be seeing this trick again in later levels 😛