Corrected OP for correct model, the K3P 12.5" barrel.
The source of my confusion on this is that everything I had been reading was that the K3P and G3K use the same bolt and carrier as the 91/91R. Then going through some older threads last night I saw a mention that the A3 stock wouldn't work with the K3P, as evidenced by HK Parts having a different stock for the PDWs. The K3P is listed as a PDW/pistol in the Atlantic website, and when I checked the HK Parts site it has a specific stock for the PDWs, and says the "regular" A3s can't be used with it and that they have specially modified the PDW stock to work.
So I've got an A3 stock with Will Springfield's heavy buffer that I got for my 91R, and if the K3P and the 91R use the same carrier and bolt, I'm not seeing why they couldn't use the same stock. I've seen numerous pics of the K3P with tele stocks and full size stocks, so the HK Parts site prohibitions makes no sense to me based on my understanding of the system.
HK Parts standard A3 stock, says not for PDW
HK Parts PDW stock, says it's for PDW models both NATO and x39
I'm wondering if HK Parts is saying PDW but meaning the sub 12.5" barrel version.