I extracted with Nymera (solo), Thora (Duo), and Lumiri (solo) but did not get credit for the extract with 3 Heroes challenge.
#3 Hero Extract Challenge
heady tundra
pastel jungle
Thank you for your report Jolly 
Ill investigate this issue!
fallow blade
Same for me. 2 duo, then left the party by closing the game, did one solo and it was 1/3 again. Didn't die. RIP 57th challenge =( Wanted to be in top 20
slate latch
My suspicion is that that challenge had to be done in the same session - so without closing the game.
I experienced the same for the ' 'Defeat 5 Changelings and extract' challenge, where I killed one in the first round and killed two and the next round, then went to sleep with the challenge at 3/5, but the next day the challenge had returned to 0/5.
hushed belfry
My suspicion is that that challenge had to be done in the same session - so with...
This challenge shouldnt be stacking as atleast for me i had to get them all in one extraction
But could be also that it works if you extracted in a row