How can I rerun cables along stairs where a wall is being removed?
If you add a box to extend the cables it will need to remain accessible. I would pull back to before the triple and drill down , it looks like there might be a level below (I think I see studs below And fire block foam where they go down?) After dropping down, route the cables back to their original termination. 
Without knowing more about the structure, that would be the best advice I could give but it may not be easy as I can’t tell the direction of the floor joists. I prefer punching through TJI’s over 2x’s when roping a house.