Appears that all three were sons of George G. and Adaline Perrin of Greene County. This is from the 1850 census.

Perrin George G. 42 SC
Perrin Adaline 42 SC
Perrin Robert 13 AL
Perrin Edward 11 AL
Perrin Susan 8 AL
Perrin George 5 AL
Perrin Frances 8/12 AL

Robert was a doctor, practicing in Pickens County, who eventually ended up as the Colonel of a Mississippi cavalry regiment [Perrin's 11th Miss. Cav.].