Three times cost is about right for large jobs, or jobs with simple text.
If quoting for vans etc you might want to go up to 4 times vinyl cost + fitting, to cover all of the phone numbers,web address, emails, street address and 'find us on facebook'. It all takes time to cut,weed and apply.
For smaller signs, unless it is a very simple design, you might need to go as high as 6 times vinyl cost + substrate cost. An 18 x 24" sign only uses 1/2 a yard of vinyl, but the weeding takes as long or longer as larger signs if there is a large amount of text. If you only charge 3 times vinyl then you are in danger of working for less than the hourly rate in Mc Donalds.