Usually wave three extends, and in commodities wave five often does. An extended wave contains a complete five-wave structure of its own, which means the same price action can be counted as three waves or nine depending on the degree you are looking at.
Extensions are where Fibonacci projections get attached: a common guideline is that wave three reaches 1.618 times wave one, measured with fibonacci-extension levels.
Extensions are also the main reason counts are so unstable. Any wave that goes further than expected can be reclassified as an extension, and any wave that stops short can be reclassified as a truncation. That flexibility is convenient for explanation and fatal for prediction.
Related: impulse-wave, elliott-wave-theory, fibonacci-extension, elliott-wave-criticism, corrective-wave