In a medium saucepan over medium heat, bring vinegar, salt and sugar to a boil. Boil until the sugar has dissolved, about 10 minutes. (I found it doesn't take that long, so I don't usually boil the full 10 minutes.)
Place the cucumbers, onions and green bell peppers in a large bowl. Pour the vinegar mixture over the vegetables.
Transfer to sterile containers and store in the refrigerator.
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