For reference:
2010 Trek 3700:
http://www.trekbikes.com/us/en/bikes/2010/archive/3700
2011 Fisher Wahoo:
http://www.trekbikes.com/us/en/bikes/2011/archive/wahoo
2011 Fisher Marlin:
http://www.trekbikes.com/us/en/bikes...archive/marlin
2012 Fisher Wahoo:
http://www.trekbikes.com/us/en/colle..._sport/wahoo/#
2012 Fisher Marlin:
http://www.trekbikes.com/us/en/colle...sport/marlin/#
Were the Marlin and Wahoo 2011 or 2012 bikes? Because both the 2011 and 2012 Marlins are 29ers (29" wheels), while the 2011 Wahoo is a 26" and the 2012 Wahoo is a 29er. The 2010 3700 is a 26". Wheel size is a personal preference thing, depending on riding style. Some guys love their 29ers and would never go back to a 26". Me on the other hand? I like my 26". Ed's signature has a link to a topic with a bunch of 29er pros and cons.
The other thing is there will be a difference in geometry between the 26ers and the 29ers, and also between the Fishers and the Trek. Ride them all, see if one feels better under you.
Assuming they all feel the same, I'd get rid of the 3700. The other bikes are a significant step up from the 3700. The only way that bike stays in the mix is if the other two turns out to be 29ers and you decide you want a 26". On the other hand, whether there's a big difference between the Marlin and Wahoo depends on whether they're 2011s or 2012s. I don't see a huge difference between the 2012s. A slightly nicer component here or there, but nothing you'll notice much. On the other hand, the 2011s have the big difference of wheel size. You WILL like one wheel size over the other...get that one.