Bobby,
I am doing this from memory as my Legacy system is at the train club I belong to. When you set up a Legacy engine you first set the ID #. I recently set up a AA set of E8 engines which should work the same as your F3s as there is one powered and one non powered. After assigning both the same ID number go into info this is where you can name your engine, assign a road number , select diesel, steam or switcher, control type TMCC, Cab-1, Legacy. Or if your engines have a orange module that will upload all the correct settings. When you set up a train in Legacy you have to do the settings manual but you can avoid that step if you set up the two A units using the same number as wel as the B unit then run them as a engine not a train. Look at it this way you only have ten train IDs but you have 99 engine IDs. Better to save the train IDs when setting up steam lash ups or mixed diesel lash ups not all F3s. The only time I use a train ID for diesels is when I want the rear unit to be reversed in the lash up. If your F AA units are like my E8s they were made to use the same ID and the auto couplers will work correctly front and rear. All you are doing different is adding a B unit which has no front so if it starts in reverse turn it around physically and it should be fine. Then address your 3 unit engine as engine # whatever and have fun.
JohnB
