What is the small white wire for on a 86 ford alternator for?

There are three wires 2 black 1 white what is the white on for

Probably ground

If it is a thin gauge wire it is probably the positive to energise the stator coil. Alternators (most of them) ground through the case. You can probably get a more certain answer on a Ford forum. I'm sure that there are many of them.

Cut the blue wire!

Ground wire for the battery light warning.