Cut the ends off the green beans and slice into pieces about an inch long. Fill a medium sized pot halfway with salted water. Bring to a boil and add the green beans to the pot and boil for 5 minute, or as long as it takes for the beans to get tender. Green beans can be highly variable in how long it takes to cook them to tenderness. More mature beans can take longer - up to 20 minutes.
Drain the green beans and put in the bowl with the pinto and black beans. Drizzle the olive oil over everything and toss to combine. Mix in the green onions, jalapeno, garlic and Cotija cheese.
In a separate bowl, mix the lime juice and sugar until the sugar dissolves - then add it to the salad. Coat the beans well and let marinate for at least 1 hour, preferably several hours. Right before you serve, toss in the cilantro. Add more salt to taste if needed.