Ooh. I'm not sure where you read that, but I'm afraid a 3 gallon container wouldn't be big enough for even a single zucchini plant. What's more, zucchini like most cucurbits hate their roots being disturbed so separating them is not really an option at this point.
I do have an idea that *may* salvage the situation to some extent though.... But how will depend on how you have this container situated and what you can do. They may still become too stressed and vulnerable to many pests and diseases that overwhelm them under best of circumstances.
What you can do is to get the vines to set down roots from as many leaf nodes as you can.
1) carefully lay the container on its side on prepared and amended ground and bury the leaf nodes
2) once the plants start vining, let them drape down to prepares/amended ground around the container and bury the leaf nodes
3) surround the container with more containers of rich but well draining potting mix and ... Well you get the idea.
I've never tried growing zucchini in containers, but a plant in the ground will get to be at least 5-6 feet across, which means they would need equivalent soil volume. I'm thinking 20-25 gallons?
I'm not sure if I would have the courage to cut and remove a plant or two, but that would be another way to give the remaining plant(s) better chance for survival.
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