I have a 1999 Buick Lesabre that atarted misfiring very badly one day (barely runs). I had a mechanic look at it and he recommended one new coil and new plugs and wires. I want to make sure the coil is faulty before i replace it so I've been following this guide
https://easyautodiagnostics.com/gm/3.8L/ignition-coil-pack-tests-5

my question is on the part where you test the switching signal. The guide says you MUST use the LED test tool and not a test light. I have a regular test light and a multimeter. Would it be possible to use one of these instead of the LED light?

This is probably because the LED illuminates faster than a traditional bulb, and the signal is a very short signal. Many incandescent bulbs don't light up fast enough to be seen doing this, but work well for longer-interval and continuous testing.

I have used a test light with no problem........as the test states, there are four possibilities for a coil not firing:

Bad coil

Bad ICM

Bad connection/wire at crank sensor or ICM...........

Bad crank sensor......

The last two are eliminated, because you have spark on the other two coils.....

If the test light flashes, it'* the coil....if it doesn't, replaced the ICM.....if the coil still doesn't fire, replace the coil.....a bad coil can take out the ICM......

I would replace the plugs and wires also, if they have not been replaced in awhile.....