Answer: Summer squash, which including zucchini, are harvested young, before they mature. This separates them from winter squash like acorn and pumpkins which are harvested when mature (seeds are fully mature and skin begins to harden). You can harvest zucchini anytime from bloom until the fruit reaches full size. If you wait too long the seeds and "meat" of the fruit will get tough and may be bitter. The same is true for cucumbers. Burpee Hybrid II cucumbers will reach 8 1/2" long by 2 1/2" thick. You can pick them prior to that if you wish; even small enough to make "gherkin" pickles!
Three seed per planting spot is fine for zucchini. They'll spread out well and may be a bit of insurance in case you should lose a plant to vine borer.
Usually green and yellow mottled fruit is an indication of virus infection. You can still eat the fruit, but the plant will decline in production and there is nothing you can do to cure it. Infected plants should be removed from the garden to reduce further infection.
The zucchini with the uneven shape is probably caused by incomplete pollination. Keep an eye on the situation and see if it improves with other fruit on the plant.
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