You sold this as a 'job lot' - that suggests multiple items - and several of your pictures show 3 items, for a cost of £15.
You wrote the listing so you will think it is clear that you will only get one. I think I would expect to receive all 3. Your listing says 'only 3 available', 'all working'.
If you sent only one drone and the buyer has opened a case through Ebay because they expected three, then you will need to provide a return label, refund the buyer when you get the item back, and then relist the drones properly: that is, separately as single, individual items.
"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)