In reply to Howard J:
> I can't see any reasons for the third person not to be prepared to assist in a crevasse rescue, and plenty of reasons for them to do so.
The third person assists simply by being there, being attached to the rope and weighing something, so they're going to assist regardless of what they're carrying.
If I was going to travel across a glacier as a three to go and do an alpine route I'd try and cut off as much weight as I could, which includes unnecessary crevasse rescue gear. So weight saving is at least one reason for the third person to not be as equipped as the first two for crevasse rescue.
Depending on the type of glacier you're crossing and route you're going to do there might be no weight savings to be had (maybe you want micro-trax's and screws for the route, so you're bringing them anyway), or there might be some (maybe you don't want microtrax or screws or a third sling on the route, so you could not bring them). A long screw and a traxion weighs about 250g along with another 200g or so for the sling, krabs and prusik. Maybe 4/500g isn't enough weight to consider saving some of it for you, but it is for some people.