Well I've flipped the 2 switches behind the glowstone doors, but I can't find any 'secret room'. I'm heartily sick of doing lab rat impressions by now, and just want to get it over with.
So where is the secret room?

YOu have to not only flip the two switches behind those doors, you have to keep the doors open -
then go back to the big "Y" shaped room - the doors and switches being at the very top of the Y and look at the very bottom of the "V" that forms the top of the Y (does that make sense??) - There is a pool of water there and just beyond it (not IN it) an alcove should have opened revealing a chest which contains the 3rd tablet.

I hate describing this room - always seems so confusing! Hope I didn't confuse you!

Wyv

So it's impossible to continue without 3 glow stones? I'm fairly sick of the stupidity already, and I just don't care anymore. Guess it's time to hit uninstall and throw it into the junk pile with all the other monuments to stupid game design. A key hunt for a key hunt? No thanks.

No - it is not impossible - you have alternatives -

You can try blocking one of the doors with a crate or something, or you can go back to the other door (not in this area) where you used a glowstone and get it back to use on these two doors.

Hope that will solve your problem!

Wyv