Old 3 wire honey with w, r and b connections. How to wire to rth2300?

Assuming b is common, c to c
R to rh and
W to w

Check out page 5 http://customer.honeywell.com/techlit/pdf/PackedLit/69-2327ES.pdf Honeywell also reminds you to ignore your old wire colors coming out of the wall but to pay attention to where each color was connected, some previous installer may not have followed the color guide.