This is such a difficult subject.
Part of me wants to agree with others that as a 3 yr old she probably has been exploring herself and doesn't really understnad what she is saying.
BUT having suffered abuse myself as a child for a number of years, I would find it very hard to just think that.
You say your DH is always very lovely to her, never shouts etc...that doesn't mean he isn't abusing her...but it also doesn't mean he is!
You need to sit down with your DH and have a good long discussion about this. He should be able to understand why you were so worried...he is her father. Ask him how he would have reacted if it had been the other way around?
You need to take her to the doctor and get this checked out. It could be just an infection. You need to rule that out.
And if it turns out that nothing has happened, then you need to remember that she is only 3 and they don't really get lying.
But (and I hate to say this) you do need to get some answers for this.
I was abused by my grandfather. He was always very nice to me...spoilt me etc etc, btu he still abused me for many years.
I'm not saying your daughter has been abused, but please don't just leave this.