You would leave the diverter valve set to the hydraulic SSQA (skid steer quick attach) and not on the 3rd function. You do not need a 3rd function for buckets or forks. The hydraulic SSQA would then function from the seat but 3rd function would not. If you need 3rd function then when you hook up the attachment hydraulic lines you switch the diverter valve. This eliminates having to run two sets (4 lines) down the loader arms.
You should never unhook an attachment from the tractor with the 3rd function hydraulic lines hooked up as you might rip them off. Having to get off the tractor to unhook the hydraulic lines means your right next to the diverter valve and you can switch it to hydraulic SSQA from 3rd function.
You would never need 3rd function until you hook up the attachments hydraulic lines. You have to get off the tractor to hook the hydraulic lines up, at that point you could switch the diverter valve from 3rd function to hydraulic SSQA.
If your moving from non hydraulic attachment to non hydraulic attachment your diverter on the loader is already set to hydraulic SSQA, otherwise you were unable to unattached the
grapple. Since the diverter is already set to SSQA it can hook and unhook attachments from the seat, until you need to operate the 3rd function.