I only get fluid out of one hose at a time.OK - it appears you want to use the selector to switch the 3rd function valve between the grapple cylinders and the rear outlets. That is fine.
The lines going to the third function should be connected to the cylinder ports .
The lines labeled front hoses should be connected to the grapple cylinders.
The lines labeled back hoses shoudl be connected to the motor ports
Is that how you have it plumbed and are you 100% POSITIVE you have the third function work ports connected to the source ports on the selector valve?
If so you when the spool on the selector valve spool is moved to select the rear outlets you should; get flow out of one one rear hose when you press one of the joystick buttons and flow out of the other rear hose when you push the other button. When the spool is moved to the other position pressing one button should open the grapple and pressing the other should close it.
Edit: Just saw your sketch. If the valve does not operate as I described in the previous paragraph you have the ports on the selector valve mixed up.
Dan
Here is a photo of the selector valve and 3rd function valve.
The 2 hoses I am holding go to the grapple bucket. The 2 hoses in front of my boot go to the rear snow blower.