There are two identical power packs on that engine, one for the 3 plugs on each head. Weak ignition components tend to bread down after the engine warms up to normal operating temps. (may fun fine when cold, then crap out when warmed up.) You have already done some troubleshooting, by indicating that 3 cyls have cut out. Coils are usually reliable, and certainly all 3 would not quit at once. So, it may be a pack. Fortunately you have a simple way to test this ignition problem. Determine which side the 3 plugs are misfiring on. Swap the two power packs. See fi the non-firing problem moves to the other head. Weak ignition components tend to start to have problems once the engine heats up to normal operating temps. So, warm the engine up well before you do your testing.
