using excel array functions on part of array

try something like this

Sub test()
Dim arra As Range, arrb As Range, rnga As Range, rngb As Range
Dim j As Double
Set arra = Range(Range("a1"), Range("a1").End(xlDown))
Set arrb = Range(Range("b1"), Range("B1").End(xlDown))


Set rnga = arra.Resize(3, 1)
Set rngb = arrb.Resize(3, 1)

MsgBox rnga.Address
j = WorksheetFunction.SumProduct(rnga, rngb)
MsgBox j
End Sub