There's two 3speed with overdrives. The first one was used from the 50's through 1965 and had a four bolt side cover with an unsynchronized first gear like this one:
http://www.ebay.com/itm/1958-1959-19...0e3a8e&vxp=mtr
The second one was used in the late 60's and had a seven bolt side cover and a synchro first gear (ie. mine). Search function on here should get a bit of posts on them.
Here's some literature if you want to read up on them. They are distinctive if you look in wrecking yards because they have an electric solenoid hanging out of the left side and a governor switch hanging out of the right side.
http://www.speedprint.com/deves50/overdriveindex.php
Hope this helps.
Regards,
Steve
New Orleans