What is causing my 2013 Holden Barina to misfiring on engine cylinder three?

If the fluid in question was fuel, you have a petrol leak and a big opportunity for the engine to catch fire. If it was water, it could be condensation from the atmosphere or perhaps even residual water from when you last washed the car. If not, the water could have come from the car’s cooling system, at which point you have a coolant leak and that needs to be fixed before the engine overheats. If the fluid was oil, suspicions would be a leaking rocker cover gasket.

Fundamentally, there shouldn’t be any unexplained fluids in the engine bay or on the engine, and if there are, something is probably leaking. By the way, the fluid you notice could have actually been the cause of the misfire; spark plugs can misfire if they’re in contact with water or, especially, oil.

My money’s on the fluid, indeed, being oil. The park plugs on this engine are located deep inside the rocker cover. Any leak from the gasket on that cover allows the oil to pool in the spark plug tube, which means it’s a fair chance this is the cause of your misfire, too.