Why create this new field for the purpose of maeking it you primary key. You can make the combination of the three fields a primary key without the need of combining them. Combining them creates redundate data. in the design view of your table, jsut select all three field and make them a primary key.
The whn you need to display you three way key as one field you can do that as a text of display purposes only.
WorkRequestID, RFANumber, and ActivityID are 3 seperate fields, and I will not be keeping them in the destination table, I will only be keeping CalcReference. For indexing purposes, surely its faster to index 1 (integer) field rather than 3? I'm dealing with a large amount of data, hence the above reasoning.