King Edward III used horses as transportation animals, not for battle. However, King Edward III did create a lighter cavalry, which was otherwise known as Hobelars. The Hobelars would use the horses to be quickly transported to the ground. Then the Hobelars would come off the horses and fight.
There were three parts in the 100-year war, the Edwardian Era, which took place from 1337 to 1360, the Caroline war, which lasted from 1369-1389 and the Lancastrian war, which took place from 1415 to 1453.
One of the arguments that took place was the argument of Gascony which was also known as Aquitaine. Gascony was geographically part of France, but it became a duchy of England. Now the people in Gascony were to pay homage to the King of England. But since France revolted against King Edward III, the duchy did not pay homage to the King of England.
In 1337, the Great Council of Paris declared that Gascony should return back to France and they also determined that it was bad to have King Edward III as their King. A reason they thought that having Edward III as King was bad was that King Edward III sheltered an enemy of France named King Robert III of Artis.
King Robert Artis had a disagreement with King Philip, and so, he fled. King Robert became a very important helper of King Edward III and even encouraged him to try to control France together with England. King Robert III also provided King Edward III with secret information of France. Together in 1340 when Charles died, they both tried to claim the throne, although they failed.
The 100-year war was a very long one that finally ended in 1453.