So would you say my next step is to check pin 3 for 12v ?I don't know if you need to crank the engine to get 12V at Pin 3. There would be no harm in having 12V at Pin 3 with just the ignition on and there are other 12volt lines feeding the ECU. The simplest thing is to check with the ignition on. If there is no reading then try cracking the engine.
I am not sure why you were getting continued to ground at Pin 3. However, if Pin 3 was at 12 Volts it may read 0 Ohms if the meter was on the Ohms range. While it should be avoided most multimeters seem to be designed to survive a small voltage when using the Ohms range.
The way ignition coils work is:
12 Volts is initially connected across the primary winding of the coil (pins 1 and 3) and then switched off. If the switch was perfect (switched in zero time) it would create a infinite voltage across the coil's primary and secondary. In practice real switches take a finite time to break the current but still generate a very high voltage across the primary and secondary windings. Transistors can switch a lot faster than the old mechanical points so electronic ignitions usually produce a stronger spark. The high voltage across the primary was a problem because it used to burn the points so a capcaitor was placed across the points to reduce the primaries voltage.
Some manufacture put the transistor switch in the coil pack while Peugeot appear to have it in the ECU.
If not there when ignition is on the check whilst cranking.