I am currently doing somthing similar.
Field1: default value 06
Field2: blank
Field3: blank set to not visible in properties
On the form: Field1 is displayed as 06, Field 2 is blank, Field 3 is not visible. You enter 009 into Field2
On the forms Before Update
[Field3]=([Field1]&[Field2])
Field3 now reads 06009
The only problem is it's saved as text.