Heat the olive oil in a large pot over medium heat.
Add the onion, garlic, carrot, and celery and cook until the vegetables begin to soften, about 5 minutes.
Stir in the diced tomatoes, kidney beans, black beans, and cannellini beans.
Pour in the vegetable broth and add dried thyme, salt, and black pepper.
Bring the mixture to a boil, then reduce the heat to low.
Simmer the stew for 25-30 minutes, stirring occasionally.
Taste and adjust the seasoning if necessary before serving.