What is the algebraic expression that best describes the sequence 3, 6, 9, 12,

The general formula is : An = A1 + (n-1) d
So, A1 = 3, d = 3
An = 3 + (n - 1) *3 = 3 + 3n - 3 = 3n.
When n = 1: A1 = 3; When n = 3: A3 = 9

Hope that helped!