I'm going to jump 3 people in line at the petrol station
Why do you want to jump 3 people in line?
It's because I have to be at work soon after.
It's rude to cut in front of people in line, because they've been waiting patiently. If everyone cut in front of each other in line it would be anarchy! In the future you may want to try to leave earlier if you want to get to work on time.
I still think it's a good idea to jump 3 people in line.
If you cut you're probably going to piss off several people.  Things could even get confrontational.  Are you sure that is a good idea?