Results from the paper

  K.G. Chetyrkin, B.A. Kniehl, M. Steinhauser:
  Strong-Coupling Constant at Three Loops in Momentum Subtraction Scheme
  TTP08-50 

(see also: 
http://www-ttp.physik.uni-karlsruhe.de/cgi-bin/webmaster/abs2html?ttp08-50)