Seriously, i think Shrek 3 is losing its appeal (at least to me).
The first episode of Shrek brought fairy tale characters onto the screen and gave them "slightly" differentiated personalities, giving people something new to watch. It was a hit probably because of the creativity.
Shrek 2 continues wth the same idea, without too much surprises, just the same old concept with more stories.
Shrek 3 is following the exact same concept without nothing new...... except for the inclusion of King Arthur. And that's almost all about it. Except that Donkey is married to the Dragon with 5 kids - flying donkeys, to be exact. The rest is still the same, Shrek is still Shrek, Princess Fiona is still Princess Fiona, both deeply in love with each other.
( A bit of spoiler - The frog king from far far away passed away, and he wants to pass the crown to Shrek, who prefers his little swamp. So he tried to get Arthur, the cousin of Fiona, to replace him. In the mean time, Prince Charming took the chance and gather the other villains to take control of the Kingdom of Far Far Away. The rest you can think of it yourself.)
The show might appeal to kids with the same old funny concept, but not to adults. It's a show for kids anyway, save your money for the Transformers, Fantastic Four or Pirates... not worth the money unless you are damn bored.