I may be mistaken, but since simple delivery was introduced it is no longer possible. With three purchases, you will be have been assigned three different tracking numbers. Some people will suggest just using one label and claiming a refund for the two unused labels. The danger there, is if the buyer claims only one item was sent in the parcel, you will have no proof you combined all three. It is a nuisance, but a lot of sellers are contacting the buyer, explaining the situation and cancelling the orders. They then create a custom listing for them to purchase again, which contains their items.
Looking at your sales, because they are of high value, I'd be tempted to just send them out separately, as you would have done if they had been bought by three different people. I'd explain to the buyer to expect three parcels. The further issue is if you combine the three into one order, make sure you then select a postal service that provides adequate compensation for your items, I think the value of your three coins greatly exceeds the cover offered by tracked 48 / 24 services. You will probably end up paying more for postage, unless you want to chance it.
