I have more than one client mostly in 2.4Ghz that are connecting not to the near AP R600 2 meters but they are connecting automatically to the far access point R500 where is 3-4 meters away plus kitchen and 2walls in between.
I reduced already the TX power of 2.4Ghz to -4dB, screenshot settings attached.
Instead of adjusting the TX Power, have you tried enabling
802.11k for the WLAN and setting a low 2.4GHz percentage in Services, Radio
Control and Band Balancing? That said,
you might also get better performance if you do not limit the radio to three