And they have reported from imam As-Sadik:

من شك في كفر أعدائنا والظالمين لنا فهو كافر

Translation:
“Who will be (even) in doubt regarding disbelief of our enemies, opressors, he is disbeliever“.

Source:
Majlisi “Bihar al anwar” 8/366
Hijazee “Duraroor akhbar” p 106
Hurr al Amili “Wasael ush shia” 28/345/#34923

Comment:
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.