Check the icon of your client, or hover over the client and it will read the name of the game.
OSRS and RS3 are two different games, and so, they have their own client
If you're thinking combat modes, then RS3 has EoC and Legacy modes. Head into game settings > combat, and you will have an option to set between Eoc and Legacy modes.
Life is like a camera: Just
focus
on what's important,
capture
the good times,
develop
from the negatives, and if things don't work out,
take another shot
!