Three-headed dog of Greek mythology — cubes err (anag) (8)

I believe the answer is:

cerberus

(Other definitions for cerberus that I've seen before include "Three-headed watchdog that guarded Hades" , "underworld guard" , "Three-headed dog guarding entrance to Hades" , "Three-headed guard dog" , "Three-headed watchdog guarding entrance to Hades" .)

I've seen this clue in The Guardian.

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