It means that the transmissions of all 3 nodes collided in slot-0 (the pre-condition for all three parts of this question), and you're asked what is the probability that all 3 nodes will successfully transmit their frame by the end of slot-3. As require each slot to be occupied by exactly one frame, it's asking for the probability that any node is successful in slot-1, another node is successful in slot-2, and the final node is successful in slot-3. (guessing that you couldn't get to any of the tutorial sessions).