Marc:
There may be a terminology problem. "Third function" to most readers means a spool valve inserted in the power beyond circuit of the FEL valve. It is almost, but not quite, the same as if there were a third spool (and related handle) on the FEL valve body to control the plow, grapple, etc. Although the third function valve can be a simple manual single spool valve, it would require a separate lever to operate it and, for most of us, more hands than occur naturally. Therefore, most third function valves are electromagnetic; that is, they are operated with a switch on the FEL valve handle (joystick). The only disadvantage (if at all) is that the reasonably priced electromagnetic single spool directional control valves are not proportional; you flip the switch one way and the plow swivels left at full speed; you flip it the other way and the plow swivels right at full speed. Most people address this problem by using small (1/4") lines for the third function valve work circuit to slow down the fluid. For a few more dollars you can install adjustable flow control valves in the circuit, or for a very few dollars you can install fixed orifice flow restrictors in the lines to the plow.
The alternate approach is to install a diverter valve (also called a selector valve) in either the lift or the curl circuit of the fel (usually the curl circuit). The selector valve is electrically operated (it can be manual, but those are awkward when you shift back and forth a lot). In one position (usually the rest position), it connects the curl spool (and work ports) to the FEL bucket, so when you move the joystick left or right it curls or dumps the bucket. But when you push the button (installed on the joystick) to activate the selector valve, it shifts the curl work ports to the additional cylinders, say the plow. So when the button is held down, moving the joystick left angles the plow to the left and moving the joystick right angles the plow to the right. The advantage of the selector valve is that the proportional flow control of the jolystick is maintained for both the original curl function and the new plow function. You can swivel the plow fast or slow, as you choose.
The commonly stated objection to the selector valve is that you can't operate the curl and the new function at the same time. Whereas, with the third function, you can activate the third function while activating the curl (as long as the curl spool is not fully shifted; that would block all fluid flow to the downstream third function valve). But there are selector valves that allow you to select either function separately (curl or plow) or both functions at the same time (curl and plow). That is, if you select both functions the curl and plow for example are connected in parallel, much like the curl and lift circuits in most FEL's. It requires a slightly more complicated switch.
I use the selector valve because I like the proportional control of the added circuit and because with the grapple it is easier to pick up objects like rocks with the curl and grapple circuits in parallel. But I am in the minority; most readers, I think, prefer the third function valve. I am sure you will hear from them on the benefits of that method.
Good luck and send pictures.
