Why is there a 3 month waiting period before converting from contractor to a full time employee at LinkedIn for some roles?
there isn't. you should be able to become an employee at any time. you can't go from *employee to contractor* for 6 months. unless there's a finder's fee in the contract they are trying to avoid. they can wait it out and then convert for free.
might be a co employment or labor law thing. not quite the same at Google though, temps must take a 3 month break every 1-2 years while remaining a temp. no break for converting