Quote Originally Posted by No brains View Post
Q 456: What happens when a warlock uses a pearl of power? Can they only cast 3rd level spells out of the recovered slot?
A456: By RAW, once a warlock is 7th level or higher and his available spells slots are above 3rd, he is no longer able to gain a benefit from a Pearl of Power.

That said, a nice DM could let you get a 3rd lvl spell slot back; which I think is reasonable and certainly not game breaking.