In a large bowl, combine the green beans, kidney beans, garbanzo beans, red onion and bell pepper.
In a small saucepan set over medium heat, whisk together the olive oil, apple cider vinegar, sugar, 1 teaspoon kosher salt and 1/2 teaspoon pepper until the sugar is dissolved.
Cook the mixture for 2 minutes then pour it over the bean mixture and thoroughly toss to combine.
Serve immediately or refrigerate, covered, until ready to serve. (See Kelly's Notes.)