Hi - well there are slings and slings. A simple front to back to carry the rifle can be used military style but then there is the single connect to a handstop and back to an upper arm loop (and optional hook on a shooting jacket sleeve).
I have used both types in the past and for prone it is incredibly stable (as in smallbore shooting) using the single sling+handstop (with or without specialist jacket but makes your upper arm sore without one). Great if the target is at the same height but very awkward when it is up in a tree, etc. Using a front focus obj. is a stretch :-) I found prone helped when I had lower back pain and couldn't use the FT sitting position but eventually I got comfortable and now can't do prone (abdominal surgery the factor there) but being so low makes wind judgment harder.
Standing with the same gear works ok but you use a lot of time adjusting handstop, etc. but there was some discussion about having to remove the sling from the arm rather than using the quick release attachment on the handstop (i.e. modifying the rifle during the course).
For kneeling I really don't think a sling should be used as it is very very difficult to keep any part of the sling form touching the arm above the wrist or forearm i.e. ISSF rules would nix it and I think FT rules would make it very suspect. I did try a few times using the sling but "pushing the rifle up" rather than letting it take the weight. Again very stable but very fiddly and just not worth the effort.
Happier putting the time in to more normal position so that I can focus on wind and hi-lo weather :-)