The 54 and 56 are the same basic transmission, both only shift once in D position.
For a true 3 speed you will have to go 58 and newer.
I added information about a 58 and newer transmission.
If you look closely you will see an over run clutch that is non existant in earlier transmissions.
http://www.oldcarmanualproject.com/manuals/Ford/1958/Service/04/FordOMaticandCruiseOMaticT/index.html