I don't think it's possible the way you mean.
You could use a long, thin cylinder entity to achieve the effect though.
You could pass the source and destination x,y,z coords to a function that creates a line entity and type, position it at the source coord, scale it along one axis to the distance between the source and destination coords and then point it at the destination before displaying it.
If your 3d line is always going to be drawn between 3d objects you don't even have to bother with 3d maths to figure out the length scaling, you can just pass the source and destination Entities and use EntityDistance.
Doing it this way means you can put some nice effects on your line and you can select and modify it easily.
Hope this is some help.
You could use a long, thin cylinder entity to achieve the effect though.
You could pass the source and destination x,y,z coords to a function that creates a line entity and type, position it at the source coord, scale it along one axis to the distance between the source and destination coords and then point it at the destination before displaying it.
If your 3d line is always going to be drawn between 3d objects you don't even have to bother with 3d maths to figure out the length scaling, you can just pass the source and destination Entities and use EntityDistance.
Doing it this way means you can put some nice effects on your line and you can select and modify it easily.
Hope this is some help.