Shia do believe that majority of companions were hypocrites and apostated after the death of prophet (sallalahu alaihi wa ala alihi wa sallam). Their scholars fabricated a lot of reports to discredit companions.
Allah Taala said in the Quran:
[Shakir 9:100] And (as for) the foremost, the first of the Muhajirs and the Ansars, and those who followed them in goodness, Allah is well pleased with them and they are well pleased with Him, and He has prepared for them gardens beneath which rivers flOw, to abide in them for ever; that is the mighty achievement.
No doubt that Abu Bakr, Umar and Uthman were from first muhajirs.
And Taala testified the faith of companions in many other verses, most known is:
[Shakir 48:18] Certainly Allah was well pleased with the believers when they swore allegiance to you under the tree, and He knew what was in their hearts, so He sent down tranquillity on them and rewarded them with a near victory,
Shia author al-Ayashi in his commentary entitled entire chapter in this way:
Chapter on the abandonment of a narration that is contradictory to the Quran
There you can find such ahadeth:
1 ـ عن هشام بن الحكم عن أبي عبد الله (عليه السلام) قال: قال رسول الله في خطبة بمنى او بمكة: يا أيها الناس ما جائكم عنى يوافق القرآن فأنا قلته وما جائكم عنى لا يوافق القرآن فلم أقله
1 – From Hisham b. al-Hakam from Abu `Abdillah عليه السلام. He said: The Messenger of Allah (صلى الله عليه وآله) said in a sermon at Mina or Makka: O people, whatever comes (is narrated) to you from me (said to be narrated by me) that agrees with the Quran then I have said it, and whatever comes to you from me that does not agree with the Quran then I have not said it.
2 ـ عن اسمعيل بن أبى زياد السكونى عن أبى جعفر عن أبيه عن على صلوات الله عليه قال: الوقوف عند الشبهة خير من الاقتحام في الهلكة، وتركك حديثا لم تروه خير من روايتك حديثا لم تحصه، ان على كل حق حقيقة وعلى كل صواب نورا فما وافق كتاب الله فخذوا به، وما خالف كتاب الله فدعوه
2 – From Isma`il b. Abi Ziyad as-Sakuni from Abu Ja`far from his father from `Ali صلوات الله عليه. He said: Halting at doubt is better than storming into destruction, and your abandonment of a hadith that you do not narrate is better than your narrating of a hadith which you have not comprehended (or, enumerated, registered). Verily upon every truth is a reality and upon every correctness is a light. So whatever agrees with the Book of Allah then take hold of it, and whatever disagrees with the Book of Allah then leave it.
3 ـ عن محمد بن مسلم قال: قال ابوعبدالله (عليه السلام): يا محمد ما جائك في رواية من بر أو فاجر يوافق القرآن فخذ به، وما جائك في رواية من بر او فاجر يخالف القرآن فلا تأخذ به
3 – From Muhammad b. Muslim. He said: Abu `Abdillah عليه السلام said: O Muhammad, whatever comes to you in a narration from a righteous person or an immoral person that agrees with the Quran, then take hold of it. And whatever comes to in a narration from a righteous person or an immoral person that contradicts the Quran, then do not take hold of it.
4 ـ عن أيوب بن حر قال: سمعت ابا عبدالله (عليه السلام) يقول: كل شئ، مردود إلى الكتاب والسنة، وكل حديث لا يوافق كتاب الله فهو زخرف
4 – From Ayyub b. Hurr. He said: I heard Abu `Abdillah عليه السلام saying: Everything is yielded (or, returned) to the Book and the Sunna, and every hadith that does not agree with the Book of Allah then it is vain.
5 ـ عن كليب الاسدى قال: سمعت ابا عبدالله (عليه السلام) يقول: ما اتاكم عنا من حديث لا يصدقه كتاب الله فهو باطل
5 – From Kulayb al-Asadi. He said: I heard Abu `Abdillah عليه السلام saying: Whatever comes to you from us of a hadith which the Book of Allah does not confirm, then it is false.
So we have a question to shias! Why did you accept ahadeth which says that companions were hypocrites, liars, kuffar, if these kind of ahadeth contradicts to the Quranic verses about companions?