I always guess that people are coming from Olympic distance, so if you are comfortable at a half iron distance, obviously 3h is not that big a deal. The difference is the mtb is not a steady sub threshold effort like a half, it is much harder, so the volume required to ride that speed is about the same as that for the half iron distance.

I think the training you are doing sounds good. Bricks are good, long rides are good, and making sure some of your long rides are on your mountainbike would be wise.

I run in a lightweight trainer or racing flat for all the races. The only time that didn't work was last year's worlds, where you needed work boots on the downhill. Most of the time a heavier flat would be good, because you are running on rocks. I use the Saucony Fastwitch Endurance or the Azura for nearly all the races.

Ciao!