So it's 20% + 20% + 20% = +60% to base shield strength. In practise the code just picks one of the modifiers then adds to it all the other modifiers - 1 each to arrive at a final modifier.
You can purchase and fit shield boosters to utility mounts.
Each booster increases your shields overall strength by a set amount when the booster is powered.
Be aware, increasing your shield strength in this manner will cause regeneration, including time required to reform after breaking, to increase.
Shield boosters are in addition to shield cell banks. Commanders should have lots of different options when outfitting, each with pros and cons.
Shield boosters specifically help to create additional choices as to what to fit to utility mounts.