Most likely there will not be any problem at the port of entry, however, be prepared to answer questions like below if the immigration officer inquires - just in case:

Why did you not come when you got the Visa 3 years ago?
Why are you visiting now? | What is the purpose of your visit?

Again, most likely their entry to the US will be hassle free, but just in case they are asked questions like these, they will be prepared to answer. Good Luck to them!