In the song "American Pie" by Don McLean who was he referring to when he said "the father,son,and the holy ghost"?

The Trinity.

https://en.wikipedia.org/wiki/Trinity

https://en.wikipedia.org/wiki/American_Pie_(song)
https://en.wikipedia.org/wiki/American_Pie_%28song%29

According to the above article, people have speculated "The Father, Son, and Holy Ghost could be:

A) Buddy Holly, Richie Valens, and J. P. (The Big Bopper) Richardson
B) John F. Kennedy, Robert F. Kennedy, and Martin Luther King, Jr.
C) The three remaining Crickets (Buddy Holly's band).

It's only speculation, but that's all we have. The singer/ songwriter Don McLean refuses to identify "The Trinity" in the song or any of the other characters ("The Jester", "The Queen", "The King", etc.)

The writer refuses to explain the lyrics, but it's often considered to mean John F. Kennedy, Martin Luther King Jr., and Bobby Kennedy.

Other possibilities are Buddy Holly, the Big Bopper, and Richie Valens; Hank Williams, Elvis Presley, and Buddy Holly; or the Trinity as earlier mentioned.
http://www.straightdope.com/columns/read/908/what-is-don-mcleans-song-american-pie-all-about