Oh dear!
Because there were 6 paintings being sold as one lot, you listed it as '6' - suggesting you had 6 sets of 6 items.
Cancel the listing now before you sell the pictures again.
Really, you should cancel the other three you have sold using the reason 'unavailable/out of stock'. However, if you do that, you will have 3 defects on your selling account.
You are also risking 3 lots of negative feedback from 3 disappointed buyers.
Personally, I would send each unsuccessful buyer a very apologetic email saying how daft I had been, explaining that I no longer had the paintings as there was only ever one set, and asking them very nicely if they could possibly request a cancellation so I could process their refund.
Maybe another mentor can come along with a better suggestion?
"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)
