TL-PA4015 is not listed anymore on TP-Link's website, so probably this model reached end of life already.

There are triple kits on the german website (see https://www.tp-link.com/de/products/details/cat-18_TL-PA4010P-TKIT.html), available for EU/UK/FR, but not for US. In general, all PA from TP-Link with 200 or more Mbps are compatible to each other, but they then use only the speed of the PA with lowest bandwidth. Therefore, I would recommend to use only adapters with the same speed (either TL-PA9xxx or TL-PA8xxx or TL-PA 7xxx) and I would buy two pairs, so one is a spare part. See also the FAQ in the support section for each Powerline adapter, there are valuable informations.

Peter M wrote

When combining PLCs of different line speeds, they don't all revert to the speed of the slowest one.

That's right. Of course, if PowerLAN adapters of the same type are talking to each other, they use the speed both are capable of. Questions was wether faster ones (TL-PA7020P) can communicate with a slower one (TL-PA4015P) and then they will use the speed of the slower one, no matter which ones are combined.