EXAMPLE.
you have the 3 letters on the skull. You don't know what they are as they are in another language.
You get the riddle which is some shit about 'I flow though a stream' so the answer is WATER.
The answer of that riddle is already given to you IN the alien language. So you know that the word there is WATER.
The alien symbol of the last letter R matches the symbol on one of the skulls! So the letter R is one of the 3 letters in the word you need!