My advice for this challenge would be to do it on the training stage (the very first soccer match you'll play). You don't play against a real computer, but you will score 3 goals during this, which count towards the total. After the three goals are scored, restart the training.
Myself, I did this on the first matchup against the computer, but it was slow and painful at times, and the goals weren't always a sure thing. I would recommend therefore doing it on the training stage. Will take around 1-1.5 hours.