Alberto Botia (Sporting Gijon), Alvaro Dominguez (Atletico Madrid) and Martin Montoya (Barcelona) had been tapped to cover injuries to players including central defenders Carles Puyol and Gerard Pique, Del Bosque said at a news conference.
"There are a few new faces on the list but this doesn't mean we will eliminate anyone from future squads," the former Real Madrid coach said.
"We have chosen this occasion to bring in some young lads and we'll have a chance to look at them more closely."
Spain play Chile, who they beat in the group phase at last year's World Cup finals, in Geneva on Sept. 2 and entertain Liechtenstein in Logrono four days later.
The world and European champions top qualification Group I with five wins from five, six points ahead of second-placed Czech Republic.
Montoya's surprise selection even wrongfooted Barca coach Pep Guardiola as the 20-year-old rarely gets a game for the club's first team.
"A reserve player being called up to the national team is a very pleasant surprise," Guardiola said at a news conference ahead of Friday's European Super Cup match against Porto. "I know that they'll treat him well and he won't feel alone."