In this paper we compute the three-loop corrections to the function in a momentum subtraction (MOM) scheme with a massive quark. The calculation is performed in the background field formalism applying asymptotic expansions for small and large momenta. Special emphasis is devoted to the relation between the coupling constant in the MOM and (MS) over bar schemes as well as their ability to describe the phenomenon of decoupling. It is demonstrated by an explicit comparison that the (MS) over bar scheme can be consistently used to relate the values of the MOM-scheme strong-coupling constant in the energy regions higher and lower than the massive-quark production threshold. This procedure obviates the necessity to know the full mass dependence of the MOM function and clearly demonstrates the equivalence of both schemes for the description of physics outside the threshold region. (C) 2009 Elsevier B.V. All rights reserved.