Three brush dynamos

On three brush dynamos, there are no regulator, only a charging relay. The output is regulated by a third brush that collects sufficient energy for the field windings. 
Sometimes there are more field windings to compensate for different loads, or, the light switch are double, one side turning the light on, the other side shorting a resistor in series with the field winding

Three brush dynamo coupled as an A-type dynamo

Three brush dynamo, couple as a B-type dynamo 

Three brush dynamos are relatively easy to convert to two brush with regulator. remove the third brush, and connect the regulator F connection to the brush holder.

With more field windings examine which is the main winding, and use this, and examine whether it is an A- or B-type dynamo, see: Which type. (third brush removed under this test)

Three brush dynamo of A-type converted with regulator

Three brush dynamo of D-type, converted with regulator

On certain systems like early Harley Davidson, help windings can be left unchanged.

Example of early Harley dynamo (1930-31) converted with A-regulator.

The second winding is connected to the lightswitch, and only adds a couple of Amps. this can left unchanged  

Later Harley Davidson use a system more like the Morgan system belowr

Example of  Morgan system, converted with  B-regulator

*If the ampere  meter is mounted in the battery  ground connection, the regulator  "A" is to be connected to the battery