H1 is more aggressive, H3 is least. It only affects the TT length, and H3 has a shorter wheelbase? That makes it more aggressive in turns.

There are many other odd things here as well, like the chainstay length for the Madone 6 and 7. Anyone else think these numbers are odd?

Here's a comparison of numbers:

H1/H2/H3 also have different head tube lengths. A shorter top tube will yield a shorter wheelbase. Doesn't look odd to me. It looks smart.

So it's just different headtube and top tube legnths. Can't that be done with different frame sizes?

seems quite middle of the road modern to me...
H3 - taller/ more upright and closer in to the seat - seems they have adapted to the consumers who are buying sport bikes with the desire to have the positions of grandma fiets...
the whole wheelbase thing is, after a certain point, more numbers gloss than real world enhancement - so what are the real advantages of shorter wheelbase - below 1000 and above ???

if anything, I find Felts to be more idiosyncratic than many others...

No. Frame sizes tend to increase somewhat proportionally. So if you get a larger frame you get a longer top tube and a taller head tube. With H1/2/3, you can get a taller head tube without a longer top tube. The longer a rider's legs are in proportion to his torso, the more head tube length in proportion to the top tube is recommended. By having multiple geometries, Trek can fit more riders more effectively without sending long-legged riders off to comfort-bike land.