Let the word of Christ dwell in you richly, teaching and admonishing one another in all wisdom, singing psalms and hymns and spiritual songs, with thankfulness in your hearts to God. (Col. 3:16)
On the surface, Colossians 3:16 seem to militate against exclusive psalmody, and its variants. Yet the exclusive psalmodists have a way around this. They interpret this phrase "psalms, hymns and spirituals songs" as essentially meaning "psalms, psalms and psalms." Of course the whole thing sounds ridiculous, yet there is evidence that make this interpretation plausible.
Now, on the one hand, the contemporary notion that "psalms" refer to the 150 Psalms, "hymns" refer to older worship songs composed by songwriters like Isaac Watts, Charles Wesley and Fanny Crosby, and "spiritual songs" refer to contemporary Christian music (CCM) is anachronistic and misguided. Yet, on the other hand, the idea that Paul would waste ink to write 3 terms for one concept sounds stupid. Of course, we have to wrestle with how the words are used in the LXX. But even if the terms are sometimes used synonymously, does that necessarily imply that one is the same as the other? Does an overlap in semantic range imply total equation of meaning? I would suggest not!
Just because some psalms are hymns, some hymns are psalms, does not mean that all hymns are psalms; that is basic set theory. That is the problem with the traditionalist interpretation of the phrase as "psalms, psalms and psalms." What is needed is not a mere proof that some hymns are psalms, some psalms are spiritual songs, or whatever uses these terms have in the 150 Psalms. What is needed to prove their case is that the Scripture teaches that there are no hymns which are not psalms, or something to that effect. Otherwise, if all that can be proven is that some psalms are hymns, it just means that the categories of "psalms," "hymns" and "spiritual songs" are not distinct categories set in stone. One could very well be singing a hymn which is a psalm (of which there are quite a couple in the Trinity Hymnal). One could claim that we are singing a new "psalm," not one of the 150 Psalms, or a "hymn" or a "spiritual song" when we sing a contemporary song with lyrics derived from Scripture.
Work has to be done on the precise manner the three phrases have been used in the Greek, but the main point here is that it is unlikely the Greeks came up with the word hymnos to make it equivalent to psalmos or ode, words which predate the LXX I might add. Colossians 3:16, whatever it is precisely referring to, should certainly include the 150 Psalms, but it is not limited to them, unlike what the Traditionalists believe.