I assumed the same at first too but its not an automatic plus +3. Speed is always limited to the slowest troop, including hero itself. So hero needs to be 19 in order for him to encourage the horses to go as fast as him.
The hero is a bucket 1L in volume. Water is the speed. You could have 5 L of water but it'll never fill all of it in the 1 L bucket. So you need a bigger bucket..
That’s definitely not how it works. It’s plus three to the slowest cavalry unit including the hero itself. A speed 14 hero with Steppes travels faster than the Steppes do by themselves, that’s easily tested. A speed 17 hero with a Spotter will travel at speed 20.