This is how The Boys Presents: Diabolical is linked to season 3

gamesradar.com:

The Boys and The Boys Presents: Diabolical showrunners have revealed how the two series are linked. 

"The finale is canon, yeah," Kripke told Variety. "I thought [Diabolical showrunner Simon Racioppa] did such a good job with it. I don’t think we had any specific plans going in for it to for sure be canon. But he just did such a good job writing and directing it, that watching it, I was like, 'This is for sure what happened.' There is a certain amount of background setup of really understanding the relationship between Homelander and Black Noir and giving us a deeper understanding before Season 3."

Homelander is played by Anthony Starr in the original series – and voiced by Starr in the animated show – and the episode follows his introduction into the Supes of the Seven and his bond with Black Noir, played by Nathan Mitchell in the live-action show. 

Racioppa added: "It’s an episode with a bit of Homelander backstory, so I wanted to make sure that felt as close to the show as possible. It has Black Noir in it. So we wanted to try and make that like a good American-action-animation-influenced episode that feels like it’s really connected to the mothership. That is the one most connected episode to the mothership."

The animated spin-off series tells previously unseen stories from The Boys universe. The anthology series is executive produced by Kripke and also features the voices of actors including Seth Rogen, Ben Schwarz, Awkwafina, Andy Samberg, and Kieran Culkin.

"Weirdly, even though it’s not at all canon, obviously, I’ll tease that there’s this kind of unexpected tie-in between Roiland’s episode and season 3. But in a way that I won’t give away," Kripke said, referring to the episode written by Rick and Morty