Appreciate the heads-up, but it appears that Trinity back-up material is being collected separately:I should note, since a few of those interested are trade-waiters, that the Trinity back-up features are not included in the Tpb collections of King's WW run. There have been two one-shot reprint specials, Trinity Special #1 and Trinity Special: World's Finest #1, where those stories have been collected. I'm sure at some point DC will do an omnibus version of the whole Tom King run which would include the back-up features, but that could be quite a long way away.

From four-year-old Lizzie Prince tormenting babysitters Damian Wayne and Jon Kent to an adult Trinity finally taking her place as her mother’s successor alongside her generation’s Batman and Superman, it’s a saga that spans decades—and light years—in adventures that set the stage for Trinity’s upcoming series!
Collecting stories from Wonder Woman #800, #3-6, #8-13, Trinity Special #1, and Trinity: World’s Finest Special #1
(Though I'm not sure why issues 1, 2, and 7 of the current WW run are omitted from the listed contents -- were there no Trinity back-ups in those issues?)