My desktop environment is Xfce.
The keyboard layouts for the three languages are:
Korean: 3-beol final
To add Russian keyboard layout in Xfce, choose Settings / Keyboard / Layout menu, then Add and select Russian (typewriter). I set the switch key to Ctrl and Shift for English and Russian.
I use Klavaro for practicing touch type to memorize Russian keyboard layout. jtsuken_typewriter matches Russian (typewriter).
uim (Universal Input Method) handles Korean and English. To switch English / Korean, I use Shift and Space keys.