nmk,
While it is certainly possible that one is working for a fool, that does not necessarily mean his request is unreasonable.
It sounds to me like the request is reasonable... at least, to the extent that you have described the situation.
You said...
I have a product that must run through 3 different but consective stations on the same machine. Each station has it's own plc.
Is there any reason that all three stations can not be controlled by one PLC?
You said...
Some times an operator might forget and skip one one of the stations(1st or 2nd stations).
Does the operator(s) insert the product (by hand) into one station, then do the operation, then remove the product (by hand) and insert it into the next station (again, by hand)?
Or, does the product move automatically from one station to the other?
In either case, can two or three stations be operating at the same time?
HAND-SHAKING:
Hand-Shaking can be accomplished through a complicated communication link (requiring all of the necessary cables and software), or it can be as simple as...
(This depends on the operational timings of the three stations. This can be done in other ways, but you have yet to describe the timing... so I can't provide the "correct" answer. This is just one way to do it...)
If it is the case that the product goes through all three stations before the next product is processed, then...
On Start-Up, an Output from Station-3 to Station-1 is turned ON.
At Station-1:
If the signal from Station-3 is present AND the Output from Station-1 to Station-2 and -3 is OFF, then it is "OK" for Station-1 to operate.
At this point, neither, Station-2 nor Station-3, can operate.
When Station-1 finishes a cycle, it turns ON one Output. That Output goes to an Input on Station-2 and Station-3. The signal from Station-1 to Station-3 causes Station-3 to turn OFF its' signal to Station-1.
At this point, neither, Station-1 nor Station-3, can operate.
At Station-2:
If the signal from Station-1 is present, AND Station-2 is NOT sending a signal to Station-3, then Station-2 can operate. When Station-2 finishes a cycle, it turns ON an Output to Station-3. While the Output to Station-3 is ON, Station-2 can not perform another operation.
At this point, neither, Station-1 nor Station-2, can operate.
At Station-3:
If the signals from Station-1 AND Station-2 are present, then Station-3 can operate. When Station-3 finishes a cycle, it turns ON an Output to Station-1. While the Output to Station-1 is ON, Station-3 can not perform another operation.
When the signal is sent from Station-3 to Station-1, Station-1 turns OFF the signals to Station-2 and Station-3. When the signal to Station-2 goes OFF, it turns OFF its' signal to Station-3.
At this point, only Station-1 can operate.
You should probably include a reset button to allow restarting the sequence without doing a shutdown.
Again, there are variations on this theme... the method that you choose to use depends on the timing.