haddoni carpets come in a variety of colors and patterns. short stubby tenticles. uniform in length. as far as difficulty, fairly moderate to easy to maintain anemones. likes deep sand and moderate to high flow.
mertens carpets have distinct orange dots under their oral disc. in more mature specimens, the tenticles around the mouth are significantly longer than the rest of them. if i remember correctly, prefers a rocky substrate, or even among rock. doesnt require intense lighting, but the stronger the better.
gigantae carpets also come with a few colors, mainly purple, green or brown. this species has the longest of the tenticles, with a distinct shaggy appearance. typically purple verrucae under the oral disc. likes rocky rubble for a substrate, demands high flow and high light. the most sensitive carpet species.
thats to the best of my knowledge of the three.
EDIT: austin...you beat me.