Reminder: spoilers...

I absorbed all three keystones before going to Lorisael and confronting the Alchemist Ruuvitar. When I went inside, I was able to use a keystone on the pedestal to open the door seals, then to free the prisoners before facing Ruuvitar, but not able to use it to do anything at the pedestal while inside the hist to heal the mnemic egg.

Was my third keystone used up somewhere else? Did I have to choose which of them to use it on? I didn't even need to use it on the door seal, since I went and explored the whole place before going in to the fight, but it was the optional objective and seemed like the thing to do since I had all the keystones and didn't want those sacrifices to be for nothing.

If you have to choose which two pedestals you want to use because the third keystone is used up some other way, is there any benefit for not using them at all and saving them for the future after the quest?

Or was I bugged somehow? Did anyone else have this happen or know whats up? I know the Khajiit said I was using the keystone to power up the big crystal I think, but it didn't seem to use it up like the pedestals do, and there was no journal warning, hint, warning from the Khajiit, or any notes laying around warning that I had to be judicious in deciding where to use the keystones now that I had them all.

I wish I had left it uncompleted so I could abandon and try again to get it to work, but I wasn't thinking. (I play for the lore and the R in the (mmo)RPG description, but I know a lot of people would just be fine with finishing the quest and moving on.)