Decoupling of c-quark loops in b-quark HQET is considered.
The decoupling coefficients for the HQET heavy-quark field
and the heavy-light quark current are calculated with the three-loop accuracy.
The last result can be used to improve the accuracy
of extracting f_B from HQET lattice simulations
(without c-quark loops).
The decoupling coefficient for the flavour-nonsinglet QCD current
with n antisymmetrized \gamma-matrices is also obtained at three loops;
the result for the tensor current (n=2) is new.
