Not sure whether I understand right all your confusion.....
But if you will start on the "Voie d'Arles" or Via Tolosana - passing through Auch -, you could perfectly well afterwards join the
Camino Frances before the crossing of the Pyrenees: in Oloron Saint Marie, you should then follow the "Voie du piémont Pyrénéen" which will lead you to Saint Jean Pied de Port, the starting point for many pilgrims walking on the
Camino Frances.
(you could of course stay on the way from Auch via Somport to Puente la Reina to join the
Camino Frances later on - or join from Oloron directly Irun to walk on the Camino del Norte... plenty of possibilities)
Three months will give you plenty of time, so you could probably start in Arles or in Montpellier and finish the whole camino from France to Santiago.
Just be aware that on the Via Tolosana, in general, it might be better to make a reservation by phone to be sure to have a bed and a supper - often proposed together in France - (well, I walked part of the route, from Montpellier to Castres, in July last year and generally, there was no problem finding accomodation).
Bon chemin, buen camino!