Drain and rinse the chickpeas, cannellini beans, and black beans well. Place them in a large bowl and let them dry slightly while you prepare the other ingredients.
If using pre-cooked beetroot, cut it into small cubes. If using fresh beetroot, cook it first, then let it cool before adding it to the salad.
Dice the cucumber, finely slice the red onion, chop the herbs, and cut the avocado into small pieces.
In a small bowl, whisk together the tahini, lemon juice, olive oil, salt, black pepper, and a little water if needed. The dressing should be creamy but pourable.
Add the beans, beetroot, cucumber, onion, herbs, avocado, to a large bowl. Pour over the dressing and toss gently, being careful not to crush the avocado.
Top with toasted seeds just before serving to keep them crunchy.
Taste the salad and adjust with more salt, lemon juice, or black pepper if needed. Serve immediately or let it rest briefly for the flavors to come together.