Please remember that the ECU in your truck was designed in 1986. This is first generation fuel injection. Removing the connector on the AFM (not a MAF on this engine) won't throw a code. Not sure what you mean by a "Lamba" sensor.
Buongiorno.
The ECU in the 3FE fires injectors in 2 banks of 3. If there was an issue with a single injector, it would likely take out the other 2 in that bank.
However, I find that VERY UNLIKELY given your description of the problem. Work through the EFI diagnostic document and remember If the CHECK ENGINE...