Left-side misfire? Some might say happens every day in the White House.
Ok, joking aside, there are four primary causes of engine misfires - a loss of spark, an unbalanced air/fuel ratio, a faulty fuel injector or fuel supply, and a loss of compression. Obviously, there are a lot of components to consider. I'd change the plugs and see if you get a repeat.
If you do get a repeat, then look for components that are common to all of the left side cyls: left side intake plenum vacuum leak, left side fuel rail, left side exhaust leak or failed cat, etc.
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