Heat 3 tablespoons olive oil in a large saute pan. Add the bread cubes and sprinkle with salt; toast bread over low to medium heat, tossing frequently, for 5 to 10 minutes, until golden brown and crispy. You might need to do this in two batches.
Place the cucumber, red pepper, yellow pepper, tomatoes and red onion in a large bowl.
For the vinaigrette: In a small bowl, whisk together the garlic, oregano, mustard, vinegar, 1 teaspoon salt and pepper. While still whisking, add the olive oil to emulsify. Pour the vinaigrette over the vegetables (not the bread). Set aside for 30 minutes for the flavors to blend. Right before serving add the feta, olives and bread cubes and mix together lightly. Serve at room temperature or cold. If making ahead of time, prepare as directed but wait a little before serving to add the bread so that it doesn't get soggy.