And they have reported from imam As-Sadik:
من شك في كفر أعدائنا والظالمين لنا فهو كافر
“Who will be (even) in doubt regarding disbelief of our enemies, opressors, he is disbeliever“.
There is no any single doubt that in accordance to shia Abu Bakr (r.a), Umar(r.a), Uthmaan (r.a) were enemies and opressors.
And that’s why we, who accept them like moslems, are disbelievers in shia view.