If all 3 labels have the same evri tracking number you would only be able to use it once. Have you been charged for 3 labels?
You could send all 3 items in one parcel but you would need to check if the weight and size of the parcel is correct for the label.
If it isn't I would cancel all 3 labels and start again by buying postage direct from Evri so that you can make sure that the weight and size match the parcel correctly.
When you have figured out the combined postage for all 3 parcels you should make a partial refund of the saving to the buyer. Otherwise the buyer would be entitled to receive 3 seperate parcels.
