With no doubt, "tri" in त्रिशूल (triśūla) is Sanskrit. You can check it in any Sanskrit dictionary, such as Apte or Monier-Williams.

Etymology of English word three or tri is

From Middle English thre, threo, thrie, thri, from Old English þreō, þriē, þrī, from Proto-Germanic *þrīz, from Proto-Indo-European *tréyes.

As per Western Linguists, PIE or Proto-Indo-European language is common ancestor of all Indo European Languages, including Deva Bhasha or Deva Vani (देव वाणि) संस्कृतम् (Saṃskṛtam). Obviously, those who know about structure of Sanskrit and its semantics will never believe in this PIE theory.