Merge documents

If I send (via email) 3 documents (bill, guarantee, coupon for next purchase ) of the same purchase (see sample below - one purchase of Nike shoes )event, how can I later "combine" them into one expense when they are already stored in the database?

I know that I can delete 2 of them, but I would like to have all three documents for later reference - so I must merge them somehow, so it would not multiply expenses 3 times at the same time I would like to have all documents stored