If using fresh green beans–rinse, end, and cut beans. Place them in a pot of water and bring to a boil. Reduce the heat to medium and simmer about 3 minutes or until just tender and still bright green. Place in colander and rinse thoroughly with cold water and set aside to drain and cool.
If using canned green beans–drain, rinse, and place in a bowl.
Drain and rinse garbanzo and canellini beans and place in bowl with green beans. Add diced celery, red pepper, onion and fennel fronds. Mix oil and agave in blender. Add vinegar and mix until combined. Pour oil mixture over beans. Marinate in refrigerator at least 8 hours, stirring occasionally.