Hi i've managed to get all three of the demons hearts from the planar sphere, however only used one to power it up. Any ideas on what i can use the other two for?

Like you obviously do, I always used to clear out the abyss level for the experience (and to put my demon-whupping cavalier through his paces). I have found no further use for the other demon hearts. I assume there were just three demons with detachable organs so that you didnt spend ages wandering around Hell with a low level party getting your buttocks mercilessly flagellated to find just one tucked away in a corner somewhere. I always just sort of kept them around as trophies for my stronghold :)

This has been discussed before and we arrived at a definite conclusion that there is no other use for the extra hearts.

You could also used shadowkeeper to place them on your hand as a ring. I did that.

Or you could mount them on large pointy sticks and parade around town with them.

Come on! They make a great stew - not even Firkraag would resist it's smell!

Here, kitty, kitty!!!