It doesn't require deduction to conclude that the bible teaches the Tri-unity of God.

To put it allegorically, if a passage says
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all normal dogs have four legs
And another says,
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Spot is a normal dog
Then it is necessarily true that Spot has four legs. The facts are contained in the book without any need for "deduction." Reformed Baptists are Trinitarian because the Scriptures contain all the statements that make His tri-unity necessarily true.

We do not find that water baptism under the New Covenant necessarily follows circumcision under the Old Covenant, and such information is not contained in the Scriptures, but rather is "deduced" from assumptions about the continuity of the Covenants. Why, for example, are female infants baptized when only males under the Old Testament were circumcised? Of course that's silly, because in Christ there are no more distinctions of gender or race. Under the Old Covenant only birth was required (and 8 days of life); and under the New Covenant it's the same: New birth. As the Old covenant was physical and temporal, so the New is spiritual and eternal. Only citizens of the Kingdom (by faith, not by the will of man) receive the outward sign of the covenant. That isn't deduced either, but necessarily contained in the Scripture.