Ah, O.K. In that case any insurer to which you transfer will almost certainly not care or even know (I wouldn't even mention that part of the discount was earned in the U.S.).
They'll ask how many years no claims you have, you'll say 3, then if/when you go ahead with the policy they will just verify that figure with your current insurer. Once one U.K. insurer has granted you so many years no-claim discount, the others will just accept it.