Question about lyrics in a song "3 From the Flatlands" involving Texas music artists?

It's kind of an obscure song from a band called " Tonk Confidential" called "I Wish You Were From Texas". They are mentioning what they like about Texas, and mention some artists, now the ones they already mentioned (and this is kind of how they appear in the lyrics) are "Buddy Holly, Willie Nelson, Orbison and Wills", and then in another part of the song they mention "George Jones and Waylon Jennings and the three from the flatlands". They mention one other artists when they say "Gene Autry was the king". So my question is, who do they mean by "the three from the flatlands"?