First of all, the horse is not "suddenly lame". The horse is responding to being injured, you know exactly when the lameness began to occur. It's not sudden.
Second of all, the horse is three legged lame, you've administered antibiotics and there's no mention of a vet?? NEVER administer antibiotics without at least a phone call to a vet. Administering antibiotics and doing an incomplete course or improper dosage, or using the wrong antibiotic all contribute to the increasing amounts of antibiotic resistant bacteria strains.
The long and the short is - call a vet. Good luck.
They say money doesn't buy happiness -- well happiness doesn't buy horses!