Each player loses a third of their life, then discards a third of the cards in their hand, then sacrifices a third of the creatures they control of their choice, then sacrifices a third of the lands they control of their choice. Round up each time.

Mystery Booster 2

Masters Edition

Each player loses a third of his or her life, then discards a third of the cards in his or her hand, then sacrifices a third of the creatures he or she controls, then sacrifices a third of the lands he or she controls. Round up each time.

Oversized League Prizes

Fifth Edition

Each player loses 1/3 of his or her life; then chooses and discards 1/3 of his or her hand; then sacrifices 1/3 of the creatures he or she controls; and then sacrifices 1/3 of the lands he or she controls. Round each loss up.

Ice Age

Each player loses 1/3 of his or her life; then chooses and discards 1/3 of the cards in his or her hand; then sacrifices 1/3 of the creatures he or she controls; and finally sacrifices 1/3 of the lands he or she controls. Round each loss up. Effects that prevent or redirect damage cannot be used to counter the this loss of life.

Each part of Pox’s effect is processed separately. For each part, first the player whose turn it is makes all necessary choices (such as which cards to discard), then each other player in turn order chooses, then the actions happen at the same time. Then Pox’s effect moves to the next stage.

The losses, discards, and sacrifices are rounded up. For example, if you have 5 life, you’ll lose 2 life, leaving you at 3 life. (Five divided by three is one and two-thirds. Rounding that up to the nearest whole number gives you two.)

The number you need to lose, discard, or sacrifice isn’t calculated until it’s time to perform that part of the effect. For example, if your opponent casts Pox and you discard Dodecapod as part of Pox’s discard effect, Dodecapod will be put onto the battlefield instead of into your graveyard. Then, when determining how many creatures you need to sacrifice, the Dodecapod is taken into account.