Boots NTR CFW.
BootNTR-Mode3 is a special build of BootNTR for running NTR with extended memory games on Old3DS.
It is no use to install BootNTR-Mode3 on New3DS since the original one works just fine.
See https://github.com/44670/BootNTR/wiki/Extended-memory-games-on-O3DS for how to launch extended memory games with NTR.