Sideboard 3 cards off of your deck. If you don't need it, don't put it in your deck. If you do use one or more sideboard card, you may use them multiple times per match. If you repeat a card, you can include multiple of that card.
Sideboard: shockwave, reverse time, freeze
G1: none
G2: shockwave - you may choose to use it once and remove it, or add it to your deck.
G3: RT, freeze - you swap to the other two, but you still used shockwave G2, so all three are discarded
Sideboard: mirror shield, mirror shield, mirror shield
G1: none
G2: mirror - if you only use one copy this match, you only need to discard one copy.
G3: mirror, mirror, mirror - You use all three at once, so all three are discarded.
Sideboard: left sock, 3.5 days, optimized stall
G1: none
G2: none
G3: none - if you use none, you discard none as well
Sideboard: shockwave, reverse time, freeze
G1: none
G2: Shockwave
G3: Shockwave - You use a single card, so you discard SW only.
Any more questions or need more examples?