Obviously lameness grading is subjective, but a 3 imo would indicate a noticable (for the average horse owner) lameness when trotted up - more than what some people call 'unlevel'. In a hind limb you'd clearly be able to see the hip dipping as the horse trots, in a forelimb there would be a noticable head nod.
Is this your horse? This level of lameness (if present for more than a couple of days) would definitely be a vet job in my book, as ime it is often something fairly sinister (e.g. soft tissue damage).
Low level lameness (1/10, 2/10, 3/10) is not necessarily better than high level. In fact a horse that won't weight bear much probably has something simple like an abscess, whereas a horse that is mildly lame could have a more serious problem like a tendon or ligament injury.
When was the horse lame, i.e. in walk, in trot, on the hard, on a surface, worse on a circle (inside or outside leg worse), worse after flexions, how did he react to hoof testers? Answers to all that should start pointing towards a problematic area. What did the vet say?
While a vetting is not a lameness examination and the vet is not working for the owner so he wouldn't try to pin point the cause of the lameness (just discover the lameness and fail the horse for it) most vets would have an idea of what might be causing the lameness. This would tell you whether it is worth treating or resting the horse and re-vetting in a few weeks, or whether it's not the horse for you.
Thanks for replies just thought I let you know what happened. Sadly we could not attend the vetting, too far away, the owner was also surprised as the horse had been hunting a few days before, The vet talked it through with us and we have spoken with the owner and as it stands we will give it 3 weeks and if the owner hasnt sold him the vet will go back and check the horse again.
That is sensible, waiting for a minor injury to recover, if it is the horse you really want, the seller was rather foolish to hunt just a few days before a vetting in my view once they are sold they are limited to how much they do until vetting because of the risk of minor incidents. If the injury is more than minor it should still show but do make sure bloods are taken.