When I went to my roster to check out this new bonus heroes feature, MPQ has been kind enough to “preselect” a 3*, 4*, and 5* for me as my "favorites 
"
I am proud to announce that I was given 3* Thor (a character I don’t use much anymore), 4* Carnage (who sucks!), and 5* Black Widow (who I’ve barely used).
So lets play a game! Did anyone actually get a random 3 characters that really are their favorites?
Thor, Black Bolt and Medusa
Thanos - who I am collecting
Wasp - HAHAHAHA. Nope.
Dr. Doom - was one of my first 3* champs. Quickly un-favorited this guy.
Now rocking:
Thanos, Green Gobin
DPX, Carol, Peggy
Squirrel Girl (Lowest level non-Thanos 3*)
Mine wasn’t bad… 5OML, 4Fury, and 3*SWitch
5* OML/ 4* hb (i need one cover for lvl 270 and another to champ) 4* red hulk/ 3* thanos -strange ( collecting)
I think i already forgot…
OML, Fury, and Doc Ock.
only one i kept was OML 
Doc Ock (champ), Medusa (0/2/2), and Widow (1/1/1)
Changed to Thanos (3/3/5), Carol (1/2/2), and Panther (0/0/0)
5* Thanos
4* Medusa
3* Captain Marvel
Now it’s
5* Thanos
OML
Phoenix
4* Cyclops
Peggy
3* Thanos
I did get a 3* Marvel as bonus before I tweaked them from an elite token.
I have not been home yet to try this new feature. How many “bonus” heroes do we get to sign up for? Is there a minimum number of 3*, 4*, and 5s? Can you choose all 4 and 5s?
Doesn’t seem to be a limit.
But the game randomly picks a bonus if you have multiples that apply.
3* - Thanos
4* - Literally every of my promoted 4s 
5 - OML and Black Bolt
Preselected:
5* - OML (always helpful, need more yellows 2/1/3)
4* - X-23 (been 5/4/0 forever. thrown away several extra green covers. can’t get any pink covers so maybe this will help)
3* - Capt America (second highest champ @ 220)
Edited:
5* - no change
4* - added 4: Peggy, Medusa, Spider-Woman, & Captain Marvel. I should probably remove some
3* - changed to Dr Strange & Iron Fist (more damage on passives)
The Pre-Selected seem to be your last 3 applied (not acquired) covers in those tiers.
My pre-selected for example were were OML, 4* Cage, Doc Ock.
My last applied 5* was OML (daily resupply) a few days ago.
My last applied 4* was 4* Cage from the last PvE a couple days ago.
My last applied 3* was Doc Ock from yesterday’s DDQ (I got 3* Thor from todays DDQ but haven’t applied it yet).
KGB
Nope, it’s completely random!
Going back to mine -
5* - OML - haven’t gotten one since around the FIRST run of Civil War, probably longer. My most recent was… Black Bolt? Or maybe Black Widow. He is my most used & most recently used 5*, but that’s not it either.
4* - X-23 - haven’t gotten a cover for her in about a month, and haven’t applied a cover to her for many months prior. Most recent applied was Miles Morales & most recent acquired was Mr Fantastic. Not used since required.
3* - Capt America - although I did get & apply one a few days ago, yesterday had both Doc Ock & Punisher.
4* Carnage doesn’t suck until you face a boosted level 370 Carnage in PvP and either get wiped or use 3 health packs when its over. Then you’ll appreciate his power as a defender. Same goes for L.C. Powerman. Face them both with Medusa and you’ll be pressing the skip button from now on.
I didn’t look long enough to see who my 4* and 5* preselected were. My 3* was Gamora, so I promptly went in and changed everything to Thanos (3*), Peggy (4*) and Doctor Strange (5*).
Then I pulled my 13th Thanos cover from an Elite token and switched that to 3* Doctor Strange so I can start trying to bootstrap him up into the 170s/180s with most of the rest of my 3* roster.
I didn’t know the game might pick for me; I rushed into my roster and picked OML, Carol and IM40.