This morning, some dragons, horses, and chickens were playing in my backyard. I counted 15 heads, 50 legs, and 4 wings. How many dragons, how many horses, and how many chickens were playing in my backyard? (For your information: any healthy dragon has three heads, four legs, and two wings. Feel free to use Wikipedia to obtain the information about the number of heads, legs, and wings of the rest of the creatures. Oh, and all the creatures in my backyard are healthy.)