I'm confused regarding Asus RT-AC57U V3 router. Is there an OpenWrt version to install on it?
The router is listed on the download page https://openwrt.org/toh/views/toh_fwdownload but there is no link to the firmware.
Does somebody managed to get OpenWrt on RT-AC57U V3 ?
This means that there is no OpenWrt support for this device (yet).
You don't want it either. The v2 and v3 have an ath10k radio which needs more RAM to operate comfortably, more than what Asus endowed them with.
See why the smallbuffer ath10k flavour was introduced: