At Serebii, people are saying that catching the 3 in-game members of the Lake Trio (NOT BRINGING THEM OVER), getting the 2 in-game orbs - Diamond and Pearl Orbs [Adamant and Lustrious] (NOT BRINGING THEM OVER), and investigating the picture are needed. Having the Lake Trio in your party was (conformed by me as well as many others) not needed nor even related.
I have posted details about the 101 preview in the thread. Check it out if you feel like it.