Speak in an unknown tongue - The Hebrew, as has already been conjectured.
Let it be by two; or at the most by three, and that by course - Let only two or three in one assembly act in this way, that too much time may not be taken up with one exercise; and let this be done by course, the one after the other, that two may not be speaking at the same time: and let one interpret for all that shall thus speak.
Let it be by two, or at the most by three - That is, two, or at most three in one day, or in one meeting. So Grotius, Rosenmuller, Doddridge, Bloomfield, and Locke, understand it. It is probable that many were endowed with the gift of tongues; and it is certain that they were disposed to exercise the gift even when it could be of no real advantage, and when it was done only for ostentation. Paul had shown to them 1 Corinthians 14:22, that the main design of the gift of tongues was to convince unbelievers; he here shows them that if that gift was exercised in the church, it should be in such a way as to promote edification. They should not speak at the same time; nor should they regard it as necessary that all should speak at the same meeting. It should not be so as to produce disorder and confusion nor should it be so as to detain the people beyond a reasonable time. The speakers, therefore, in any one assembly should not exceed two or three.
And that by course - Separately; one after another. They should not all speak at the same time.
And let one interpret - One who has the gift of interpreting foreign languages, (Note, 1 Corinthians 12:10), so that they may be understood, and the church be edified.