Sandoval had described Rayo's battle to stay in the league as "going to war with a water pistol" last week due to the restraints on his budget so he is delighted to add some much need depth to the squad.
He told Efe: "The three of them have opted for this club knowing I can be a shop window for them, but they have also identified with the fans who have received them very well and it is always important to feel loved by them."
Armenteros was part of the Rayo squad last season as they achieved promotion to La Liga and Sandoval is particularly pleased to have the Argentinian back at the club.
"The arrival of Armenteros is one of moves I had asked the club for and the have granted it," he said. "I think he is a fundamental piece in this project, the shame is he wasn't here from the start of the season but sometimes it is better to wait."
Sandoval knows his side face a tough task this weekend as a confident Athletic Bilbao visit Vallecas.
He said: "Athletic have been one of the surprises of the year and they don't seem to be wearing down. They are putting in supernatural efforts in three competitions and we have to really believe in ourselves and play an intense game if we are to pick up a very important three points."
All three new signings could feature for Rayo with Lass Bangoura still away at the African Nations Cup and Dani Pacheco out injured.
Athletic will be missing one of their key performers in recent weeks as Oscar de Marcos is suspended following his red card against Real Madrid last weekend, but Jon Aurtenetxe should return at left-back after his ban.