In Frogs, the oxygenated and deoxygenated blood is not exclusively separate bacause of the three-chambered heart. The circulation itself is highly ramified. Frog also have a large shallow abdominal artery and from this artery a lot of blood vessel near their skin highly branch into cappillries to serve the gas exchange via the skin naka. I think this is enough to compensate for the RBC's less efficacy in carrying oxygen.