They say a 3 foot drop COULD ruin a helmet, however the helmet type can play a big part. Is it DOT or Snell? Snell is a harder helmet that does not absorb impact well, it is more likely to crack, whereas the DOT helmet is a softer material that would probably be fine after a low speed drop in the grass.
Is there any visible damage? I would check into sending it back to the manufacturer if you are unsure. Most companys will inspect it for free if you are willing to pay for shipping both ways. I would be a little leary about taking it to a dealer for inspection unless you really trust them. for one they aren't neccescerilly expert in helmet engineering (Neither am I though) They may recomend replacement just to sell you a new one or they may actually recomend replacement for the safety and liability reasons.
Only you know how hard it hit the ground so ultimately you have to make the decision!
Hope I was of some help.
Good Luck, have fun and be safe.
Steve