The equation for damage taken is: 1 / (100% + Res1 + Res2 + Res3 + …) so even if all resistances together were 200% you’d still take 33% damage (= 1/300%). It’s impossible to reduce damage to 0 except with a shield that gives outright immunity (Transformer, Red Suit).

There’s no quick way to calculate this because most resistance stats the game gives you aren’t what’s put in the formula, but the end result – assuming you’re using only one at a time. For example, this should be true for the shield:

The actual formula value for the shield is probably 180%, so damage taken is:

DmgTaken = 1 / (100% + 180%) = 1 / 280% = 35.7%

The resulting damage resistance thus is

100% - 35.7% = 64.3%

which is rounded and stated on the card as “64%”.

Similar reverse engineering is required to figure out the class mod’s formula value before putting it in the formula. To make matters worse, the game is inconsistent with how it gives resistance stats, the artifact value is probably the actual value in the formula.

Overall, I think you should get a total incendiary resistance of about 70% with these three items. After the big resistance from the shield, further returns are drastically diminished.

I explained it in more detail in the post that Steeveyb linked.