There are 2 types of fuel injectors "high impedance" & "low impedance", the megasquirt 3 can drive the high impedance injectors without the versafueler.
Theory is that the low impedance injectors are better than the high when using BIGGER injectors. Mine are 160lb Bosch low impedance that require the use of a resistor wired in-between the negative or triggered side of the injector when using the MS3. You can make your own resistor packs but I don't believe they will work as well as a contained unit like the versa fueler. The versafueler also is supposed to help the bigger injectors have a much better off idle response than using a high impedance injector. Idle quality is supposed do be way better also.
You can view the wiring diagram in the above post to understand how it gets wired in-between th ECU and the injectors.
