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I would imagine most Protestant theologians who believe in the Trinity believe that the concept is in scripture.

Indeed. It is in the Scripture, but explicitly. The tri-unity of God is one of those doctrines (like Sunday worship instead of Saturday) that is not directly and explicitly stated or commanded in Scripture, but is nonetheless necessarily true. Here's what I mean by "necessarily" true:

This is a very simplistic example, but it's clear. Let's suppose that there is some verse of Scripture which explicitly states, "all normal dogs have four legs." And let's suppose another one explicitly says, "Spot is a normal dog." In that case, we don't need another verse to tell us that Spot has four legs. We already know that. In fact we are forced to that conclusion. It is necessarily so.

The WCF calls that "good and necessary consequence."

-Robin