Hi Susan,
First this explanation:
"Exemestane is an irreversible, steroidal aromatase inactivator"
http://www.rxlist.com/cgi/generic3/exemest_cp.htm
the key differences there between it and the other two (anastrozole and letrozole) is that it is "irreversible" and "steroidal". To get the entire picture, talk with a good pharmacist about it.
I don't have a package insert for Aromasin handy (maybe someone else does here) but when I read it a while back it was listed as a "second-line" treatment, whereas Femara and Arimidex are listed as "first-line" treatment for breast cancer for postmenopausal women.
Arimidex has the longest history of all 3 drugs in figuring out how well it works. That doesn't mean it works better necessarily, just that they have had more time to know more about it.
Generally either Arimidex or Femara is prescribed first, even though Aromasin is sometimes better tolerated.
AlaskaAngel