What part of the game, hun?
No there is not a third crest in phantom hourglass.
the hourglass itself
its a trifource
you draw the Zelda symbol
the triforce which is this thing
Draw a hourglass on the red door
which crest if you mean the triforce or hourglass, you have to draw it on the door, if you mean the sacred crest, it is past the first red door, the one you draw the hourglass
u write 7 on the little thing next to the door
If you mean a door with the wisdom sign on it then you need the wisdom fairy's power to open the door.
Check the related links
beam the 3 statue lasers on the temple door
they are holes that bombachus can go through and usally hit a swicth or open a door