Summer Succotash

6 ounces fresh cranberry beans, shucked
4 ounces each haricots verts and wax beans, cut into 2-inch lengths
6 ounces sugar snap peas
3 garlic scapes, cut into 1-inch lengths (optional)
2 ears of corn, husked
1/4 cup plus 2 tablespoons extra-virgin olive oil
3 tablespoons champagne vinegar or white-wine vinegar
1/4 cup fresh tarragon, chopped
Coarse salt and freshly ground pepper
1 avocado, cut into 6 wedges, then cut crosswise into thirds
5 ounces grape tomatoes, halved (1 cup)

Prepare an ice-water bath. Cook cranberry beans in a large pot of salted boiling water until just tender, 15 to 20 minutes. Transfer to ice-water bath. Drain. Repeat with haricots verts, wax beans, and sugar snap peas, cooking together for 2 minutes. Repeat with scapes if using, cooking for 1 minute. Repeat with corn, cooking for 2 minutes. Cut kernels off cobs.
Whisk oil, vinegar, and tarragon in a small bowl. Season with salt and pepper, and mash in 3 avocado pieces. Place cooked vegetables, remaining avocado, and the tomatoes in a large bowl, and toss with dressing. Serve immediately.