Engine running rough

I have a V6-24V HJ. I started it up after not running it for a few years. In preparation I had bought new spark plugs and leads (I think you call them 'wires' in the US) because I remember it was running on only about 4 cylinders just before I stopped driving it. I changed the front three plugs and leads (because they were easliy accessible) and then fired the engine. It started but it runs rough. It seems to be running on 4 cylinders still.

I pulled all the leads in turn and it seems there is no spark on two of them and they are on the same 'coil pack?'. Don't know if that is the right terminology. Any ideas on rectifying this issue?

Incidentally, in case anyone asks, no I didn't drain the fuel tank as I couldn't see a drain plug and there was less than a 1/4 of a tank in it. I just filled up on good fuel (gas). I'll change fuel filter after going through this tank.