Avoid the A75, its OK for very short sections, but its a very fast bit of road with a lot of traffic. Both of your other options are a lot nicer. The Corse of Slakes has a big climb (cat 3 I think) and is quite narrow, but you get a great view from the top. The B796 goes past the old railway station and is still a fair climb, but is not as bad as option 2, its a nice bit of road, recently resurfaced, and really enjoyable.
Which direction are you going btw? I usually ride from Gatehouse to Creetown via the railway station (option 3) then come back via option 2.
I’d say it depends how much you want to hurt yourself which option you take. Option 2 is about 320m of climbing vs option 2 of about 190m.