Illia Zabarnyi has acted as the senior centre-back, with Marcos Senesi or Dean Huijsen rotating alongside him.
Iraola told the Daily Echo: "I'm very happy with the performances of the three.
“Very happy. And even Hilly (James Hill) is the fourth one, when he played against West Ham, he was very good.
“But these three that have started the Premier League games, I think they've had very good performances and it's not easy for me.
“I suppose it's not easy for them because they will all want to play. I try to find the partnership that I think will give us more chance in every game.
“Sometimes it's not because they played last game better or worse, because they fit better with the demands of the particular game and they have to continue pushing and giving me reasons to select them like all the others.”