Company claiming i did not return all goods so won't refund me

[ArtisticMeeg]

I ordered some work related goods from a company I have used many times before in early December. It was a buy 2 get 1 free offer. Realised when I received them they were not what I wanted. Emailed company who said no problem here is a returns form and label to send back. So I did that.
After 2 weeks had heard nothing so checked the tracking and it had definitely been delivered 2 days after I sent it. Emailed them and they said we have recieved your 2 items for refund and it will be processed in 1-2 working days. I wrongly assumed the "2 items" was stated as the "free" one was irrelevant to the refund. Still nothing after 5 days so I emailed again. Today I recieve an email saying they only recieved 2 of the 3 items so until I send the last one back they won't refund me.
But I DID send them all back. But obviously I cannot prove this no more than they can prove I didn't send them. What can I do here? I have replied saying I most definitely did send them all back and why are they only telling me they apparently don't have them over 2 weeks after they recieved the returns. I'm not sure what I can do next without any proof either way.

Was the parcel weighed? Might that help show there were 3 things returned?

That's a good idea. I can't remember but I still have the tracking reciept thing so hopefully it might be on there?

So you sent all three items back and they are saying they only received two?
Did you pay via credit card? If so take it up with your bank. You could try and talk to someone higher up in the company and say you definitely sent all three items back, and when they said they had received two, you assumed they weren't counting the third, and do they not have systems in place to show stock control of products in and products out and products returned.
You could possibly tweet them.

Yes it is what I assumed at the "recieved 2" email especially as this was followed with you will recieve your refund in 1-2 working days. And I said this in my reply today to them. At no point in that first email did they say they had not recieved the 3rd one so would not refund me. Plus they didn't originally contact me to say they had only recieved 2 even though my returns form said 3. I've had to chase 3 times to get them to state they haven't recieved all 3. I've used this company before (and would happily have continued to use them) and had no issues so I'm quite surprised that this has happened.