I concur with Scott Semple’s comments. In particular: These kinds of questions are not of the yes/no type. They are highly individual. You need to make these sorts of decisions based on your own (not someone else’s) experience. Here are a few guide lines I’d offer.

If you are bonking at 3 hours then eat something. Bonking is not a desirable energy state to fall into because you will be reduced to walking and the training effect will be greatly diminished.

There is a balance between fasted training vs recovery and fasted training vs performance. If you train in a fasted state to cause extra glycogen depletion you WILL improve fat adaptation. But there is a very good chance you will extend the recovery time between workouts. This is why we recommend fasted training during only Z1-2 workouts during a period where performance or delayed recovery is not going to have a serious downside. We have a little saying that we use: “Train on fat. Race on carbs”. By race we mean any important event where performance really matters.

In the end you will need to play around with the balance mentioned above. Some days it will be appropriate to dig way down into glycogen depletion (sans bonking). Other days you’d be wise not to.

I hope this helps.
Scott