Class 3 generally means that you have to use your hands, as well as your feet, to make upward progress. As a general rule, climbing down is more dangerous than climbing up, in terms of slipping.
If you have no rock climbing experience, Whitney can be a problematic place to have your first. The issue is what is referred to as "exposure", or air under your feet. This can really get to a person who is not used to it, and can be paralyzing. Some people like to have a rope for the last 400 feet, and guided groups routinely set them up, at least in winter.
The other danger is rockfall. The two major areas are in the chute climbing out of Iceberg Lake, either naturally falling from the mountain, dislodged from climbers on the East Ridge route, or (grr) people throwing things off the summit.
The other spot is in the last 400 feet, where rocks can be dislodged above you.
You should really have a climbing helmet if you are going to do this route.
There is also a responsibility, if you are doing this route, to know what you are doing, with respect to those around you. You should know enough not to be dislodging rocks onto others below you, you should be able to move fast enough that you don't create a bottleneck at critical times, which could cause problems for a lot of people. You need to know where you are going.
If you have not climbed the mountain before, I'd advise the regular route, particularly if you have no rock climbing skills. It would be good to at least get some experience in a rock climbing gym and learn basic rock moves before doing this, it will make it a lot simpler trip when you do it.
But the Mountaineers Route is not a trail, it is a cross country route that requires navigation (more than Langley), basic rock climbing skills, decision making, combined with endurance, and all at 14 thousand+ feet, which really makes it a scramble.
Whichever way you go, have fun, and post a TR!
