So my question is, is it right for a sunni to deem of christians who believe in the trinity as polytheists? what scholars agree with this? is there a consensus?
Depends on what you mean by 'polytheists'. The Arabic term is Mushrikeen and regarding that Ibn Hazm (384 - 456 هـ) wrote:
واتفقوا على تسمية اليهود والنصارى كفارا. واختلفوا في تسميتهم مشركين
(The scholars) are agreed on calling the Jews and Christians as Kuffar. And they differed on calling them Mushrikeen
And al-Razi (544 - 606 هـ) wrote:
اختلفوا في أن لفظ المشرك هل يتناول الكفار من أهل الكتاب، فأنكر بعضهم ذلك، والأكثرون من العلماء على أن لفظ المشرك يندرج فيه الكفار من أهل الكتاب وهو المختار
There is difference on whether the word Mushrik is used for the disbelievers among the People of the Scripture. Some have denied it and most of the scholars are of the view that the word Mushrik includes the disbelievers from the People of the Scripture and this is the preferred view.
Al-Kasani (587 هـ) wrote:
لأن أهل الكتاب، وإن كانوا مشركين على الحقيقة لكن هذا الاسم في متعارف الناس يطلق على المشركين من غير أهل الكتاب
For the People of the Scripture, even though they are Mushrikeen in reality, however this name is commonly used among the people to apply to the Mushrikeen other than the People of the Scripture
And there are many other such examples. Even at the time of the Sahaba, it is reported that Ibn Umar (died in 73 هـ) applied the word Mushrik to the Jews and Christians. And there are ahadith which apply the word to Jews and Christians and there are verses in the Quran where it is possible to interpret that the word has been especially applied to Jews and Christians such as:
وقالوا كونوا هودا أو نصارى تهتدوا قل بل ملة إبراهيم حنيفا وما كان من المشركين
They say, "Be Jews or Christians [so] you will be guided." Say, "Rather, [we follow] the religion of Abraham, inclining toward truth, and he was not of the polytheists."
The evidence for not applying the word Mushrik to the Jews\Christians are verses which mention them separately such as:
إن الذين كفروا من أهل الكتاب والمشركين في نار جهنم خالدين فيها أولئك هم شر البرية
Indeed, they who disbelieved among the People of the Scripture and the polytheists will be in the fire of Hell, abiding eternally therein. Those are the worst of creatures.
The argument is that if أهل الكتاب were included in مشركين then it would be redundant to mention them separately. And that Mushrik is not clearly applied in the Quran to the Jews and Christians even though they commit shirk.
However this argument is not widely accepted. Rather there are counterexamples such as 2:98 where Gabriel and Michael are mentioned separately from the angels, 55:68 where dates and pomegranates are mentioned separately from the fruits, and 33:7 where Muhammad, Noah, Abraham, Moses and Jesus are mentioned separately from the Prophets.
مشركين is used in a variety of meanings in Islamic texts:
It can mean disbelievers in general, in which case Christians are among them.
It can mean those who associate others with Allah. Trinitarian Christians are among them since they believe that Godhead is composed of three distinct persons. This has been called shirk in the Quran in 5:72, 9:31 etc. And Mushrik is simply one who commits shirk.
It can mean idolators exclusively, in this usage the Christians are not among them.
This is something that piqued my curiousity as I had understood that Christianity was monotheistic. So I was startled to learn that in Christianity that Christ, the Holy Spirit were seen as aspects of God. In fact, during the early history of Christianity there were fierce clashes on the exact relationship between the three with eventually the trinitarian formula settled in Christianity.
Christians, from the beginning of Islam were seen as people of the book, and so not polytheists.
I didn't read much about older scholarly view of People of the Book beliefs, but the wording of their calling is clear from the Qur'an itself.
Notably, the Qur'an is very specific when distinguishing the polytheists from Christians & Jews, in case of the Jews they have agreement [with Muslims] that Trinity is a work of polytheism.
However, Allah doesn't call Christians "Polytheists" even when He remarked their imitation of polytheists in Qur'an [9:30], the Trinity is indeed polytheism, it was in existence in ancient pagan religions before Christianity.
But the reason behind calling Christians "People of the Book" instead of "Polytheists" is explained:
29:46 ۞ وَلَا تُجَـٰدِلُوٓا۟ أَهْلَ ٱلْكِتَـٰبِ إِلَّا بِٱلَّتِى هِىَ أَحْسَنُ إِلَّا ٱلَّذِينَ ظَلَمُوا۟ مِنْهُمْ ۖ وَقُولُوٓا۟ ءَامَنَّا بِٱلَّذِىٓ أُنزِلَ إِلَيْنَا وَأُنزِلَ إِلَيْكُمْ وَإِلَـٰهُنَا وَإِلَـٰهُكُمْ وَٰحِدٌ وَنَحْنُ لَهُۥ مُسْلِمُونَ
And do not argue with the People of the Scripture except in a way that is best, except for those who commit injustice among them, and say, "We believe in that which has been revealed to us and revealed to you. And our God and your God is one; and we are Muslims [in submission] to Him."
So the moral:
- Don't call a Christian polytheist, since he/she can get the impression that Muslims are worshiping a different god.
- Don't attack a Christian by the Bible corruption, biblical scholars are aware of this fact, but testify you believe in what's true in their books.
- Don't argue a Christian without knowing their beliefs, clashing in debates with them will have bad consequences if not executed properly.