I am not aware of any 3 foot narrow gauge in Madagascar. The two main railroad lines in Madagascar are meter gauge, 3ft 3 3/8". There was a meter gauge system on the adjacent island of Nossi-Be that had three H.K. Porter 2-6-0s of about 1919 vintage. It would seem unlikely that these would be the Army Porter 0-6-0s you are referring to. Since Madagascar was a French colony, most motive power was European, although there were a few Baldwin 0-6-2Ts.
For some time I have contemplated doing a Madagascar narrow gauge posting, so maybe I will do that sooner rather than later, now that you have made a mention of that fascinating island