VB.net SQL

I have 3 Tables -   Accounts, Stock, Customers
Accounts - Fields
Invoice,Name,Edited

Stock - fields
Stocknumber, name, edited

Customers - fields
Customerno, name, edited

I want to run a sql to create a table all String fields to have
Field1,field2,field3,Field4

accounts then
field1 = ‘A’,field2 = Invoice, Field3 = Name, Field4 = edited

Stock then
field1 = ‘S’,field2 = Stockno, Field3 = Name, Field4 = edited

Customers then
field1 = ‘C’,field2 = CustomerNo, Field3 = Name, Field4 = edited

something like
Select
‘A’ as Field1 (if stock then Field1 = 'S')
,a.Invoice as Field1
,a.Name as Field2
a.Edited as Field3
from Accounts as a
then how do you add the other table data without losing the accounts data