To get triple MP, you need an ability that costs 3 mist charges. The third Quickening is the most common choice, but the espers Famfrit, Chaos, Ultima, and Zodiark also work. Giving Fran Mateus and two Quickenings will only result in her having two abilities costing 1 mist charge (Mateus, Feral Strike) and one costing 2 mist charges (Whip Kick).
I tested this just now to be sure. Basch with Belias, Mateus, and a Quickening only has one mist charge, and adding Hashmal and a second Quickening to this gives him a total of two mist charges.
I tested this just now to be sure. Basch with Belias, Mateus, and a Quickening only has one mist charge, and adding Hashmal and a second Quickening to this gives him a total of two mist charges.