It is said that the three Lions came into existence through the reign of Richard I, the Lionheart, around the turn of the twelfth century, as the official Royal Coat of Arms of England, and remained so for the next 140 years.

When Richard I came to the throne, so too did his personal coat of arms.  The three lions appeared in gold on a red background. Before this point, only two golden lions had adorned a red crest, following the Norman Conquest of 1066 (the House of Normandy).  Then following the succession of King Henry II (the House of Plantagenet) in 1158, it became one golden lion. Differing stories will have it that Richard's lions were based on the original Normandy arms, with an extra lion added to represent the ongoing Anglo-Norman alliance.  Another story will tell us that two leopards were combined with another from Aquitaine, a region in South-West France, on the acquisition of more territories to the crown. Yet another version could be more simple, King Richard simply brought together the lions of Normandy and Plantagenet, and created the new Three Lions. 

Now to throw a spanner in the lions cage, being obsessed with all facts around the Battle of Agincourt, I read somewhere that Henry V had a coat of arms displaying three leopards during that campaign. Can anyone confirm further details of this? It's a worry!