killing dogs

i have 3 boston terriers and i have one i have owned for 16 years and i have two rescues that i have owned for 2 years my oldest is a male and the others are females and every now and then they get into a fight and both females attack my old male so i got home after work at 1 am tonight and cant find my oldest pup finally i located him under my tralior and rip out skirting to find him half alive i am currently sitting with him debating on ending them bitches lifes but i love them too and nature is what it is so anyone got any input on this i would appreciate it