In that culture when you cited a passage that everyone knew, the whole passage was directly implied and understood by everyone, just like if I give you a meme you know the unstated implications of the picture or brief words. The passage in Moses says two or three witnesses directly, and Jesus cited that exact passage but used the application about two witnesses. So Moses said it and Jesus meant it.

It's also significant that the part of 1 John that is verified says the Spirit (Jesus's Breath), the water, and the blood are three witnesses, citing the same passage. (I think Tertullian wrote the other part, the "Johannine comma", in a margin, and then it got added to many manuscripts afterward due to misunderstanding.) So what we do have is that Father and Son are two witnesses (many Jesus-only types accept this), and Jesus's Spirit is also another witness ("another" can mean of the same or of a different category, and most followers of Jesus are fine with this if it's a different category).