In a bowl, whisk together 1/2 cup Italian dressing, 1/2 cup BBQ sauce, 1/2 cup bottled ranch dressing, 1 tablespoon chili powder, and 1 tablespoon red pepper flakes until smooth to form the marinade.
Place the 6 boneless skinless chicken breasts in a large zip-top bag or shallow dish. Pour the marinade over the chicken, press out excess air, and seal.
Refrigerate the sealed bag for at least 2 hours or up to overnight to marinate and absorb flavor.
Prepare the grill (or preheat a cast iron grill pan) to medium-high heat.
Remove the chicken from the marinade and discard any remaining marinade. Place the breasts on the grill and cook, turning as needed, until an instant-read thermometer inserted into the thickest part reads 165°F, about 12–15 minutes total depending on thickness.
Transfer chicken to a plate and let rest a few minutes before serving.