I would take the parcel to the Post Office (or to the courier you plan to use) and buy a label direct.
Then, you can claim a refund for the three labels Ebay will charge you for.
Do inform the buyer that you are sending the items together and give them the correct tracking number.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)
