As any force in a plane can be resolved into two orthogonal components (X and Y), similarly any force in space (3-dimension) can be resolved into three orthogonal components (X, Y and Z)

If there are multiple 3-D forces acting at a point, they all can be resolved into respective components.

When all the X components of the forces are added, we get the X component of the resultant, also called as $£Fx$ (summation Fx). Similarly we can get $£Fy$ and $£Fz.$

The magnitude R of the resultant can be found from

$R^2= £Fx^2 + £Fy^2 + £Fz^2$

And its direction cosines will be

$l= £Fx/R; m= £Fy/R; n= £Fz/R$