Anthony is quite right and I got myself confused thinking 72.5 was steeper than 73. So I should have written:

Bike 1: Reach is X
Bike 2: Reach is X plus 5mm
Bike 3: Reach is X minus 10mm

The head tube angle differences also affect the reach as Anthony says, but the actual effect is small (probably less than 1mm). The seat tube angle makes a much bigger difference because the assumption is that the saddle is moved (either on the rails or via a setback seatpost or combination of both) in order to keep the rider in the same preferred position relative to the bottom bracket. So, with a slacker seat tube angle, more of the top tube length is "used up" reaching back to the seat tube, leaving a shorter length in front of the BB to affect the reach.

The geometry works out such that one degree steeper in seat tube angle equates to roughly 10mm increase in reach, if everything else is kept constant.