Mysterious Lady,
The first question is what kind of bike do you want? The bikes you listed are what I call "performance hybrids" and are best suited for hard surfaces.
The Gary Fisher Mendota and the Trek 7.5FX are essentially the same bike except that the Mendota has disc brakes and is priced a couple of hundred dollars higher.
The 7.3FX has an aluminum fork (instead of carbon), is 8 speed (instead of 9) and is priced a couple of hundred dollars lower than the 7.5FX. In my wife's opinion, the trigger shifters on the 7.5FX are much better than those on the 7.3FX.
If you want a performance hybrid, all three are good bikes. Fit is the first priority, followed by condition and personal appeal. Price is .... negotiable.
Just my opinion, YMMV.
Hope this helps.