Wounded Stray

We have had this stray cat in our neighborhood for several years. Won't get near humans but I have seen it eating our cat's food if no one or no cat is around. A neighbor noticed it last week end walking/limping on 3 legs; either wounded or hit by car. We tried to catch it but no luck. I left some food out thinking he did not have it to fend for himself. Any suggestions of how to catch it and have it put down out of mercy? I have given St Francis a call to intercede for this poor cat. Mary