Text of sermon 3 or sermon shaqshaqiyah of nahjul balagha is here https://www.al-islam.org/nahjul-balagha-part-1-sermons/sermon-3-allah-son-abu-quhafah
Sources of this Sermon
Allama Baqir Majlisi in his historical work Bihar ul Anwar narrated all the versions of this Khutbah Shaqshaqiyah from all primary shia hadith books along with some other reports in the chapter named:
باب شكاية أمير المؤمنين صلوات الله عليه عمن تقدمه من المتغلبين الغاصبين
"Chapter of Ameer al mumineen's PBUH Complaints about those who preceded him from the oppressors who took his right"
Online Source to this chapter from Bihar ul Anwar: http://ablibrary.net/فارسی/کتابها/7645_بحار الأنوار(29)/صفحه_545
Authenticity of this sermon from Shia Scholar and Researcher of Bihar ul Anwar Shaikh Asif Muhsini.
Asif Muhsini in his "Mashara'at Bihar Al Anwar", said:
اورد فيه المصنف ( 69 ) شاهداً من الاحاديث والروايات ، والمذكورة برقم (17) معتبرة سنداً فقط لا غير ]
[ مشرعة بحار الانوار / محمد اصف محسني ج 2 ص 34 ]
"the Musannif(Al Majlisi) has placed in this chapter 69 narrations, only that narration which is number (17) has an acceptable chain of narrators" [Mashra'at bihar al Anwar 2/34]
So the only reliable narration out of all 69 narrations in the chapter is 17. Which in fact is not sermon of shaqshaqiyah. Check from online source of Bihar ul Anwar Here
Below is the scan of what Shaikh Asif Muhsini said in his "Mashara'at Bihar Al Anwar".
Some more thoughts on this fabricated sermon.
Jazak Alah khairan.