3* red shards for warriors

[Hypnos]

I have 4/6 but there aren't any on the drop info of each mode on any of the maps? Where are they possible to obtain? I have underworld madness unlocked but I'm thinking it might just be from the secret locations..

[Dutch]

i have tons of those, try toys of death in battlefield madness, or the first couple in underworld hard where you can just auto battle through it

[Vaseodin]

Yes, both of the ones Dutch mentioned gave me tons of reds.