Each of the stunts CAN be done at the 1-Star driving skill, but the Long Jump is extremely difficult. No matter the speed or the vehicle I would use at the lower levels, I could not get the Long Jump. I HIGHLY advise getting the 3- or 4-Star skill levels before attempting these.
I also advise using the Agency tunnels, but I would also highly recommend the Beach on La Mugre, near the amusement park. There are four beach houses, each with ramps on the front of the building that are SUPERB for launching off of.
Additionally, on Los Muertos' upper island, they have a very large coliseum with an indoor stunt track. AND, if you don't have the Buggy, there are two ramps angled specifically FOR doing barrel rolls. Nice.
BARREL ROLLS - I recommend the Coliseum, Beach, Tunnel #2. Tunnel #1 isn't bad either, but it's fairly easy to run into the wall before you have the chance to complete it. If you have the expansion DLC with the extra vehicles, use the Buggy. You'll get it REALLY easily. If not, use the Agency SUV.
FLIPS - Again, I recommend the Coliseum, Beach, and Tunnel #2, but Tunnel #1 is excellent for the back flip too. I'd recommend avoiding Tunnel #1 for the front flip, though. It's a little harder. Again, use the Buggy if you can, but otherwise, Agency SUV is best.
LONG JUMP - Beach, Tunnel #1, Tunnel #2, or Tunnel #3. I couldn't execute this for the longest time. Finally, I tried out the Agency SUV with 4-Star driving skill. I headed down Tunnel #1 to La Mugre, and as soon as I was about to ramp out the tunnel, I hit B to jump. If you copy this out of ANY of the tunnels OR on the Beach, you will SOAR. When I did it, I jumped clear across the highway and through to the intersection up the hill on La Mugre. Without doing the SUV jump, though, I couldn't get it to register at all, and I tried seriously EVERYTHING I could come up with. The Buggy is normally the best for jumping without the 4-Star rank, but I couldn't get it for anything with that. If you have trouble with this one, get the 4-Star rank THEN come back with the SUV. You'll get it in no time.