How to update column values in a table where that column is referring to different tables

i have a table Order it has columns

OrderID, Name, Total

1            trade   value populated from i.e. tradeorder table

2            fixed   value is populated from fixedorder table

3            annual  value populated from another table

I m thinking of creating function for every order id to call those tables..

and there are many insertions taking place in order table ,,at the moment its 20 rows ..

kindly provide your suggestions