What is wrong with my dog's leg?

We have 3 boxers and they are always rough housing and hurting each other "boxing" . They have come inside limping on several occasions sometimes its their nails from digging or clawing at each other. If nothing looks swollen and it doesn't seem to hurt to bend their legs I let them limp around for a few days If they really look like they are in pain I give them an aspirin If it last longer than a week I will take them to the vet. With 3 of them I can't afford to take them to the vet for every broken toe nail or pulled muscle.