Hi Dave,
1/2/3. With intervals your goal is to accumulate a certain duration in a certain power bucket. This is easier to achieve on the road or trainer. 2 sets of 3 x 3 should accumulate 15-18 minutes in power L5.
Training with intervals is best done on the road or trainer to hit specific power targets. Endurance, tech skills and race pace training is best done off-road on terrain similar to where you will race.
Coach Lynda