Well, you could see damage reduction as the result of damage resistance piling on. I keep the base damage at 100 for the simplicity.
20% damage resistance equals 100/(1+0,2) = 83,33 damage (so 16,67% actual reduction)
20% damage reduction is actually just 20% less damage, so you receive 80 damage
The last one is potentially tricky, but I just assume that it gets calculated together, which would make it 66,66 damage. Or a third less damage.
Overall, wether it is called damage resistance or damage reduction, taking less damage is always a good thing. If the description says damage reduction however, you can basically just assume that it reduces the damage by that percentage.
Look no further as to Amara’s “Helping Hand(s)” skill. At 5 skill points you get 40% damage reduction out of it, which means from 100 points of incoming damage, Amara receives just 60. Damage reduction is much more straightforward and the “honest” way to describe taking less damage (the numbers behind the scenes aren’t important for us gamers, so the term “damage resistance” shouldn’t even need to exist in the game, as it isn’t a clear indicator of anything if you don’t know the math behind it).