Spark plug one is the best, spark plug three is the worst, and spark plug two is the middlest.

There is a z shaped hose entering the plenum chamber near cylinder three, if this hose is cracked, excessive unmetered air could be entering the system, and so the impact would be worst on cylinder three. This might be the cause of the problem.

Since the picture posted upside down (although maybe it’s showing correctly in Australia?) the order left-right on your screen (with the motorcycle on the ceiling) is 3-2-1. Same answer?

balance your air intake (throttle body) AFTER installing clean injectors. Reset TPS.

Spark plug one is the best, spark plug three is the worst, and spark plug two is the middlest.

There is a z shaped hose entering the plenum chamber near cylinder three, if this hose is cracked, excessive unmetered air could be entering the system, and so the impact would be worst on cylinder three. This might be the cause of the problem.

What are the spark plugs' model numbers, and what do the electrode gaps measure?

A full oil replacement involves removing the axle and wheel then removing a screw at the bottom of the slider by means of an access hole in the slider on the bottom of the axle mount. It is usually done after disassembling components from the fork tube for inspection or repair. That screw can sometime rotate instead of loosen from the assembly; loosening can take more finesse.  An oil change for a Showa would take 410cc; a full replacement would take 420cc.

For your diagnostic purposes, draining oil from the bolt hole on the side of the slider is much quicker because wheel and axle aren't removed. Good quality ATM is acceptable as replacement fluid, some experienced riders here seem to prefer it; regular fork oil ranges from 5W-10W depending on your body and cargo weight, and the most common road conditions you encounter. 10W seems to be what was used when new.

Sometimes, fork oil has never been changed on these relics so drain bolts can be stubborn to loosen. I heat them, apply Liquid Wrench, and wait until the smoke clears. I tap on the face of the bolt head a couple of times after which it usually loosens with a crack when torque is quickly applied via a six-sided socket on a ratchet handle or by an electric impact wrench. Don't use a dying strain on the ratchet handle to loosen them; the bolts can break or the bolt heads can be rounded. Repeat the heat-and-quench instead.