The reason a Triumph gearbox will jump out of gear could be due to one of any number of faults, or more usually to a combination of more than one
Fitting a new camplate plunger and spring will certainly help but is rarely the real reason the gear won't stay in. If you eventually strip the box inspect every single item of the selector mechanism becuse it can all wear, and wear means that the movement of the gear lever isn't being translated into a corresponding amout of travel at the selector forks.
The camplate on the 3HW 'box is made out of some sort of monkey metal and the inner tracks can open out quite dramatically, it can also strip away from it's shaft, which is steel, and the mating teeth with the quadrant can wear. If any of these conditions are present replace the camplate. Also check for excessive wear between the camplate shaft and the steel bush in the casing.
The selector rod can wear which in turn makes the selectors twist on operation - the same one was used on all pre unit 'boxes so it's an easy fix.
The selector fork ends can twist (due to wear in the preceeding components) or just plain wear out, as can the pins for the camplate rollers and the rollers themselves. They can be reclaimed, but it's not an easy job. Unfortunately the gearbox was redesigned for 1950 and the selectors are not interchangeable so new ones are a bit thin on the ground.
At this point all you're left with are the gears themselves which are usually indestructable when used on the 350. A quick look will tell you if something is amiss.
As an aside, another weak point on these 'boxes is wear on the floating bushes between the layshaft and the layshaft high gear. This can sorted by hard chroming the layshaft slightly oversize and reaming the bushes to suit - funnily enough, there's rarely any wear between the bushes and the layshaft high gear, which is a plus.
Hope this helps,