This example enters the number 3 as an array constant in cells A1:C5 on Sheet1.
Worksheets("Sheet1").Range("A1:C5").FormulaArray = "=3"
This example enters the array formula =SUM(R1C1:R3C3) in cells E1:E3 on Sheet1.
Worksheets("Sheet1").Range("E1:E3").FormulaArray = _
"=Sum(R1C1:R3C3)"