It's about right for Fielden. Anyone would be angry abut having their leg stamped on, but contrary to what some are saying, he did not 'defend himself'. It wasn't a sustained attack and therefore there was no reason to defend. What he did, was retaliate to the incident and attack Keith Mason after the incident had happened. Some might argue that he was justified.
Mason should have got eight matches for that stamp. He could have snapped Fielden's leg, it was deliberate and nasty. Three matches is a joke.