They all have increased surface area for gas exchange - fish have gill filaments covered with lamellae, frogs are small to maintain a high SA:V ratio and use their skin and basic lungs, while lungs in mammals have extensive branching.
They also need to be kept moist. No problem in fish, while frogs always live in moist environments and need to stay wet. Mammals have internal gas exchange surfaces and mucus to maintain moisture.
There are also many capillaries that service the gas exchange surfaces, with very little diffusion needed, only two cells (gas exchange surface and capillary wall).
Not sure if you could also add that at these surfaces, oxygen diffuses into the blood while carbon dioxide diffuses out...