1. Bring a large pot of water and 2 tablespoons salt to a boil. Add green beans and cook until just tender (5-7 minutes). Transfer with a slotted spoon to an ice bath, drain, and pat dry.
2. Add lima beans and black-eyed peas to boiling water and simmer until tender (16-18 minutes). Drain and pat dry.
3. While limas and peas simmer, whisk together lemon juice, pepper, remaining 1 1/2 teaspoons salt, oil, and scallions.
4. Toss green beans, lima beans, and black-eyed peas in the dressing. Serve warm or chilled.