Downn 3 cylinders

Single mag, probably bad distributor on mag. Pull the distributor plate where the wires go in and check the distributor for cracks and broken springs. Grease it up good with dielectric grease and see if you get more cylinders firing.
Once the plug wire cap is pulled off, inspect the insulator for carbon tracing of each dead cylinder, then inspect the distributor block for the same thing, I'd suggest replacing the insulator, you may have to pull out distributor block and media blast it with fine glass bead, if there are cracks in the block, replace it as well, could be from leaking oil seal, or moisture.
In either case, you want it all clean, dry, and no oils, or grease of any sort on insulator or block, a thin coat of flange sealant at the mating surfaces of the wire cap and case for good measure is fine.
Don’t neglect the plug wires as a potential major contributor to the problems you are having

Don’t ask me how I know

Some people gotta learn everything the hard way
I am one of those people!