Make sure you already set Anna to her Slut Route first, then choose to do her Sharing Route with both Adrian and Josef.
Then, go upstairs on Wednesday and Josef will invite you to do hiking together ( this event is pure RNG, you can exit to main menu or progress through the next week to try again if you didn't trigger it the first time ). Accept his invitation, progress through several dialogues and Anna will try to get you, Adrian and Josef all worked up and you end up doing gangbang with Anna (Together Trophy Unlocked)