Misfiring cylinder3 '99 Toyota Avalon how to troubleshoot?

Hello, a very common problem for cylinder misfires are the ignition coil packs, remove the spark plug and replace the plugs if necessary then replace the coil pack if the misfire persists, the #3 cylinder it at the cylinder head near the firewall and is the one in the middle of the head.

Just as a hint to diagnose ignition coil. note what cylinder that has misfire code. in your case its cylinder 3 located in a tight spot under intake. swap out number 3 coil with number 4 coil. (which is easier access ) and run vehicle if code changes from P0303 to P0304 then the coil is the issue and replace it (also easier to get at now). if misfire stays at cylinder 3 then further diagnosis is required.

I’m happy to assist further over the phone at https://www.6ya.com/expert/james_9ddabbbc483e2add