K-Line MP-15 Reversing Units

I can tell you a little about the series parallel switch, it is used to run the motors in either series or parallel wiring set up from the factory. Series will give you the slower speed.

Lee Fritz

The Series/Parallel switch changes the internal wiring to the motors. Model train motors are normally wired in parallel. Switching to series cuts the voltage to each motor, causing the locomotive to run more slowly, giving more realistic operation for a switcher. 

It sounds like your units may have different reverse boards. There are different ways to set up a 3-position E-unit. The traditional way, like that of old mechanical E-units, is that on a cold start the unit starts up in the next position from when it was shut down, so if it is shut down while in the forward position, next time it starts up in neutral, then when the power is interrupted it goes to reverse, etc. The other way is for the unit to start in the same time on every cold start, i.e. when the power has been off for several seconds. Units of this type usually start in forward; old QSI and Proto-Sound units start in neutral. Some units can also be set up to start in reverse for the rear unit of a multi-unit setup. 

Sounds like you have two traditional-style E-units and one that starts in forward. The only way I know to run them all together would be to lock them all in forward, which is possible if all three have a switch to lock out the E-unit. You could also change out the one non-matching e-unit for one that matches the others (although actually, changing all the e-units to "start forward" types would give you an easier setup to operate). I have a bunch of K-Line E-units I could sell, but since I got them used I have no idea what type they are. 

There may be other solutions, which if they exist I will leave to our resident electronics wizards.