First, you don't know it He did or not, because a lot of what was taught tom the Apostles was not written down. Second, the whole Church practiced triple immersion form the beginning, is it was not a matter of private interpretation of a local church but something accepted by all churches.
The fact that they used a word that has implications of multiple immersion does not mean that it is automatically so (much less 3), and as Greek speakers they would have been aware of this possible interpretation
The number of immersions represents how many days Christ was dead before he rose again. Baptism involves immersion (burial). For obvious reasons, one cannot remain submerged under water for three days, so the three-day submersion burial is fittingly represented by three immersions before the newly baptized person is resurrected.
If the word baptiso means multiple immersions, what number would be fitting for the person being baptized to follow in Christ's footsteps if not three? Obviously, one is not correct because the formula calls for multiple. Two is meaningless. So is four, and five, because they have no bearing on Christ whom we are following in death and resurrection.
You’re right, a lot of what the Apostles taught may not have been written down. But in their letters to various churches, they addressed the most important aspects, as well as the most common heresies. Would God have allowed something vitally important to salvation to be forgotten? The authors may have been men, but the words of scipture are breathed from God, and He is a reliable source.
If the number of immersions was vitally important, would they not have put that down?
We are not aware that the whole of the early church practiced triple immersion. Our source material here is limited, but as far as I’m aware, the number of immersions was not a matter of contention.
I agree that the most logical formula would be three immersions, and this is entirely valid from both a logicl and symbolic level... but is salvation a matter of formulae and ritual? Does salvation depend upon men following a ritual?
Or is salvation dependant entirely upon God? If it is dependant upon God, is it ritual and the symbolism/ritual of men that is important, or the spirit and truth which are written in our hearts?