If you have 3 chaingrings and 8 cogs that gives you 24 gears. You can count the teeth on them all and then look them up on a gear chart. Or just shift on the other chainrings and learn by doing.

What he is warning you against is "cross chaining".... Being in a gear that forces the chain to go from the most inboard cog to most outboard chainring or vice versa.

Shift into the middle chainring and easier cogs for uphills, experiment.