Applying a formula to Get From Web URL with VBA Macro

I can see 2 problems:
obvious one - sum in VBA
more complicated - double quotes. probably web contents expects string enclosed in double quotes. so to start such string you need 3 doublequotes - first is just "this will be string" signal, next 2 will be interpreted as one doublequote
Try:

Source = Xml.Tables(Web.Contents("""http://www.thisurl.com/" & application.sum(1,2,3) & "/restOfTheUrl"""))

or

Source = Xml.Tables(Web.Contents( chr(34) & "http://www.thisurl.com/" & application.Sum(1,2,3) & "/restOfTheUrl" & chr(34) ))