You can actually fly on track and do SLOP at the same time, at least in NAT, since you are recommended to fly 0,1,2 or 3 miles right of track. Most of the flights I choose to SLOP 0 miles.
You might want to brush up on the procedure. The "O" in SLOP is when you are flying 1nm or 2 nm right of the assigned track. The assigned track is and always has been the primary route. SLOP allows for two additional strategic procedures that do not normally require any additional clearance.
3nm is a new one on me?