What is the next step I should take to undrain my toilet?

So, I'm trying to get this straight in my head. You have two toilets u/strs., one down. If you have two out of three toilets working fine, and you've tried all the methods you described, my bet is that the toilet you are having problems with is the last one in the plumbing line, (the farthest one away from where the drain line goes out of the house). So, somewhere between that toilet and the next toilet in the line there's either a partial plug or a plugged vent going up through the roof. You didn't mention vents, so I have to think that that may be the problem. If you have access to the roof, and aren't afraid of heights, find the vent that services that last toilet, drag a garden hose up there and stick it down in the vent as far as you can get it to go, then have someone turn it on. These vent pipes get clogged up occasionally with leaves, small twigs, etc. and sometimes even an old bird carc will pop up out of there. They lite on the edge of the tube, and maybe fall, maybe get blown over, for whatever reason they wind up in that small pipe and can't fly so they can't get out. That may just fix your problem. If it doesn't, then I guess I'd go for the plumber.