In many early manuscripts (and commentators), the section “in heaven: the Father, the Word and Holy Spirit; an these three are one. And there are three that bear witness on earth” does not appear.
True....but, we should note that the Greek textual line which started the deletion of the Comma was perpetuated by Arians - who would have had a perfect reason to delete this passage - who more or less controlled the Eastern (Greek) portion of Christianity for around 50 years.
At the same time, we also see the Comma being cited by early Western (i.e. Latin) writers such as Tertullian and Cyprian. Augustine also later alludes to it as well. The Latin evidences suggest the presence of the Latin in the early pre-Vulgate Latin translations, which would have been made from very antique Greek mss.