Each individual row on the treble side of a typical three-row button accordion is in a single key, with the possible exception of a few notes at the top (chin end) of the keyboard. The rows are a fourth apart. The most common combination is G/C/F, popular for playing many of the varieties and permutations of Tex-Mex music.
The layout of the rows is the same as on two-row, non-Irish-style diatonics; please refer to that page for an explanation.
If you want to play traditional fiddle tunes, if you want to own only one button accordion, and if you don’t mind a slightly larger instrument, an A/D/G is a reasonable choice. This is especially true if you’re playing with others and so are expected to play the tunes in the keys in which they were written.
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