Fennel, Grapefruit, and Apple Salad

2 bulbs fennel, trimmed and thinly sliced

½ red onion, thinly sliced

1 green apple, peeled and thinly sliced

1 teaspoon lemon juice

1 red grapefruit - peeled, white pith cut off, and sectioned

2 tablespoons chopped toasted walnuts, or to taste

1 tablespoon torn basil leaves, or to taste

2 teaspoons lemon juice, or to taste

2 teaspoons lime juice, or to taste

2 teaspoons grapefruit juice, or to taste

¼ cup extra-virgin olive oil, or to taste

salt and ground black pepper to taste