Given: equally spaced x values, x0 till xn and respective function values in two columns

Three points n=2: Simpson's 1/3 rule
Function Simp13 = (x2-x0)*(f(x0)+4f(x1)+f(x2))/6

Four points n=3: Simpson's 3/8 rule
Function Simp38 = (x3-x0)*(f(x0)+3f(x1)+3f(x2)+f(x3))/8

Five points n=4: Simpson's 1/3 rule on first and last three points

Six points n=5: Simpson's 1/3 rule on first three points and Simpson's 3/8 rule on last last four points

etc.