Hi Brown Owl,
The EV3 route is roughly based (in France at least) on the Voie de Tours (or Via Turonensis) pilgrim route - which has a sub-forum on this site. But it isn't the only way to cycle towards St Jean Pied de Port and the usual start of the
Camino Frances.
Two years ago Carol and I cycled our tandem from Mont St Michel to Santiago. We were briefly on the EV3 (from Aulnay for a day or so) but chose the Voie Littorale/Velodyssee - along the coast west of Bordeaux - to Bayonne, then cycled up the Nive valley to SJPdP.
Alternatively, there is a Cicerone guide to cycling from Le Puy to Santiago, not directly on the walkers path, but frequently intersecting it and potentially sharing some of the same pilgrim-focused accommodation.
On the EV3 in France, you will encounter a fair number of other cyclists, but most of them won't necessarily be interested in the pilgrimage, since it in effect joins together several short local/departmental cycle routes.
Once in Spain, the EV3 is mostly undeveloped and unsignposted - it's more of an aspiration than a reality. To continue towards Santiago, you will need to decide whether and when you will follow the walkers' path and when you will take the road - there are several recommended routes out there, depending on your bike (mountain bike or road bike), the season, your desired speed and your willingness to share the route with walking pilgrims who won't be (nor should they be) always ready to jump out of your way.
I'd be interested to hear what you decide.
Bon chemin/bon camino!
