I was rubbish at all sport at school, except for middle & long distance running both road running and cross-country). I later spent a dozen years refereeing youth and adult football matches (on pitches in vastly different conditions), with plenty of road running as part of my fitness training. So, based upon that, here's my answer:
If it was very short grass on firm ground (e.g. a school running track in a hot, dry summer) you'd probably hardly notice any difference in your time.
Running in a park, on slightly softer ground might add 2 or 3 minutes to your time.
Running up and down a muddy football pitch (in a state where the referee would find it a 'borderline' decision as to whether it was actually 'playable') would add around a third, so perhaps 5 or 6 minutes.
If you really want a challenge though, try running on a beach with soft sand. I'd expect you to take at least 30 minutes to cover 3 miles, if you could manage it at all!
Chris