Should Muslims believe God is a Trinity?

Introduction

“Yes, after all…

The apostles believed that God was a trinity

The apostles of Jesus believed that God was a Trinity. This is relevant because, if Islam is true, Jesus was a prophet of Islam, and the apostles teachings reflected his.¹

But so what? Plausibly…
(a)… the apostles were fooled.[See answer]²
(b)… the apostles were simply mistaken.[See answer]²

Saying the apostles were fooled or mistaken runs into the following problem: Jesus's apostles were victors who rose to dominance because of Allah's support for them, indicating their message was approved.
(1) There were victors rising to dominance who Allah supported. Sura 61:14 -- Jesus… said to the disciples, "Who are my supporters for Allah ?" The disciples said, “We are supporters of Allah” And a faction of the Children of Israel believed and a faction disbelieved. [Allah then says he] “supported those who believed against their enemy, and they became dominant.” [Sahih International]
(2) The victors who rose to dominance who Allah supported were either Jesus's apostles or not Jesus's apostles.
(3) If they were not Jesus's apostles, then we would see records of these non-apostle victors.
(4) We do not see such records.
(5) Therefore, it is false that the victors who rose to dominance who Allah supported were non-apostles.
(6) Therefore, the victors who rose to dominance who Allah supported were Jesus's apostles.