Some citations for further research:

Combat Data and the 3:1 Rule
T. N. Dupuy
International Security
Vol. 14, No. 1 (Summer, 1989), pp. 195-201
(article consists of 7 pages)
Published by: The MIT Press
Stable URL: http://www.jstor.org/stable/2538771
Assessing the Conventional Balance: The 3:1 Rule and Its Critics
John J. Mearsheimer
International Security
Vol. 13, No. 4 (Spring, 1989), pp. 54-89
(article consists of 36 pages)
Published by: The MIT Press
Stable URL: http://www.jstor.org/stable/2538780