As mentioned earlier the satellite communications are received by the INMARSAT service which uses geostationary satellites covering much of the earth. Here is their latest map of coverage for the service (Classic Aero) which covers ACARS.
Our coverage - Inmarsat
You will see that there is only one satellite covering the India Ocean (the blue one) so that will have picked up the signals for most of the morning of March the 8th (until just after 8am).
So there will be no triangulation involved, and not even two satellites to work off.
Obviously the plane did not turn up under the coverage of the western green satellite because then the authorities would be more definite about the final signals.