Finding average for any number of rows

Hello,

So i have three coloumns filled with data, which can have any number of rows. i want to build a macro that will find the average of the three columns in other column (the fourth column next to them). then have the macro autofill until the last row with data in it. it needs to be an excel macro with the application im working with. here is what i have so far and please feel free to ask me any questions.


Sub avgtest()
'
' avgtest Macro
' Macro recorded 7/1/2009 by Administrator
'

'
v = [E65536].End(xlUp).Row
b = 3
Range("F3").Select
ActiveCell.FormulaR1C1 = "=AVERAGE(RC[-4]:RC[-1])"
Range("F3").Select
Selection.AutoFill Destination:=Range(Cells(b, 2), Cells(v, 5)), Type:=xlFillDefault
End Sub