Alcacer has scored 14 times in 31 games for Valencia this season, while Parejo and Bernat have also been key performers in the second half of the campaign at the Mestalla.
"The development of Alcacer reinforces the opinions of everyone who has backed him for many years," Del Bosque told Mundo Deportivo.
"[Parejo] came to Valencia as a great player and now he is playing very well. He's one of the team's strongest players. [Spain] never used to have any left-backs but suddenly Jordi Alba appeared, and we decided to play him.
"Before that we had none and now we have Alba, Alberto Moreno, Nacho Monreal and even Juan Bernat."
Del Bosque will name his final squad next month.