Is Trinitarianism Biblical or pagan?

The Biblical justification for Trinitarianism (or possible lack thereof; it depends on how you interpret certain passages) is one of the oldest and most debated subjects in Christendom. However, any Trinitarian church would reject your assumption that the Trinity implies tritheism. The common statement, "One God, in three persons", may not be intuitive, but it's what the vast majority of Christianity has believed for centuries.

Your first question is probably too broad. Your second question's answer is: no.

Pagan religions often have various Triads of gods, but a Triad is not a Trinity. For example, there is Anu, Enlil and Ea, the three gods of Mesopotamia, many centuries before Christ. But they were not a Trinity; they were a Triad because they were three distinct gods. Some might claim that the pagan Greek goddess Hecate is a Trinity but that error is dealt with in this link: http://soloontherocks.tumblr.com/post/65357485016/the-greek-deity-hekate-is-not-a-triple-goddess

Hinduism has a Triad of deities, but it does not have a Trinity. The Hindu triad is called 'Trimurti'. Let me quote from a Hindu scholar on this:

"Sometimes the three gods Brahma (a personification of the impersonal Brahman), Vishnu and Shiva are grouped together in a triad called Trimurti, occasionally although wrongly described as a 'Hindu trinity'. In this representation of 'One God in three forms' Brahma is the creator, Vishnu is the preserver and Shiva is the destroyer. Well known from Indian sculpture, the triad has largely remained artificial and is unimportant to Hindu worship, which is much more strongly founded on the notion of istadevata, the idea that an individual or a family chooses a specific god as the main focus for devotion." Article on Hindu triad in Encyclopedia of World Faiths, Ed. Bishop & Darton, p 193 Dr Ursula King (Macdonald Orbis 1987)

Now, the differences; the Trinity is the one Being of God subsisting in the three Persons of the Father, the Son and the Holy Spirit. The Godhead is personal, not impersonal. All three Persons created everything, all three Persons resurrected the incarnate Son, including the Son himself, all three Persons are in unity and work together sustaining the universe and so on. The Godhead is worshipped. And to worship the Son or the Holy Spirit is the same as worshipping the Father, for they are not three gods, but the One. None of the three Persons were ever created. No female deity was involved in the Godhead. This One God abhors polytheism, the worship of more than the one true God.

Those few differences alone show critical differences between the Hindu triad of Trimurti and the Christian Trinity of the Godhead. For more on the question of whether the Christian Trinity is pagan, see this link: http://www.bible.ca/trinity/trinity-pagan.htm

To deal with your question, "Is the concept of trinitarianism biblical?" would require many pages of quoted Bible texts. Many ancient Hebrew scriptures and many Christian Greek scriptures can be cited as the reason for the Trinity doctrine, yet without need to resort to 1 John 5:7 (the verse you quote in your question). Many other verses were being marshalled by the early Church Fathers long before the Trinity doctrine was formulated at the A.D. 325 Council of Nicea.

That Council had been called to bring an end to a dispute that had been gathering momentum since the end of the first century A.D. – it had culminated in Alexander and Arius disagreeing on the person of Christ. It was Tertullian (A.D. 160-230) who did major spade-work to express Trinitarian belief a century before Arius. The Nicene Council formed a creed that would prevent people like Arius wheedling their way around statements of belief so as to maintain their positions in the Church, when they were actually teaching heresy about Christ. The Council was proof that the debate had been going on for some 250 years.

The Council decreed that anybody who could not agree with this formula would be anathema – cursed: it said that the Son of God was “true God of true God, begotten not made, of one substance with the Father.” Trinitarianism was implicit in Christianity from the beginning; it is only its explicit formulation that took a long time to develop.

For scholarly explanations of how the three Persons of the Godhead can subsist in the One Being of God with no tritheism involved, see 'Heresies and Orthodoxy in the History of the Church' by Harold O.J. Brown, pp 128-130 (Hendrickson 1998).