I felt the same way.The first 2 I really enjoyed. The third I felt got a little cartoonish in the violence at times, but with that said I still thought it was pretty good.
In the first 2 he seemed extraordinarily skilled/gifted/experienced, etc. With his fighting abilities.
But in the third movie his stamina, strength, and endurance had reached superhuman/superhero levels. If his character was ever "believeable", it wasn't anymore. A few times the fight scenes dragged on and seemed repetitive. But as you said, still fun to watch.