Season three, the episode is called "Just Harried."
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Piper and Leo tried to marry 2 times first in "Magic Hour". They are caught by the elders though and Leo is taken away. The elders allow him to come back in the episode "Once upon a time" this time they are allow to marry if they can proof their relationship is strong enough to survive. Finally, they really marry in Just Harried.
I do not know the name of the episode but I am pretty sure there is at least one episode were Leo tickles Piper.
Leo and Piper get married in the third season and I think that its on the fourth disc. The episode's name is "Just Harried."
Piper finds out in 6x16 and Leo in 6x18
On the episode "A Call To Arms" Piper receives extra arms making her have 8. In this episode Piper and Leo receive powers of Hindu Gods. This is the first episode of season 7.