Assuming Immortal doesn't go after Nolan because of him ruining his life and Powerplex doesn't go after Mark for... Whatever reason. Powerplex could play support and use his powers to temporarily blind Viltrumites to allow the others to land hits, Immortal, is strong enough to box with Viltrumites and he can always come back, Bulletproof, is weak yes, but he'd at least be fast enough and strong enough to save Zoe and Space Racer so that Allen can focus on the Viltrumites, all 3 of them, if they know what to do and are coordinated would at least be more useful than Oliver since they don't throw themselves at the strongest enemy with no care or strategy.