Those symbols refer to EuroVelo Routes (EV), a set of long distance routes spanning Europe or following a theme.

They are normally made by linking various local routes together and different maps may display the route under different names.

You'll find the EV3 route already created in sections by country in CT at the bottom of this page 

For example, the CT page for the Spanish section is here 

The EV website: https://en.eurovelo.com/ev3

And the Spanish version for some Spanish practice :-) https://www.eurovelospain.com/rutas/eurovelo-3/

Buen Camino!