3PK user here. Is everything still function properly? I mean all the pots are still in good condition and has no oscillation? If so I don't see a reason to upgrade.
Still, I've got Flysky NB4 recently and it's a step up in term of response (I ran Spektrum module in my 3PK). Apart from that, not much. NB4 has almost the same control layout as 3PK too with 2 dials on the grip so it's very easy to get used to.

By the way what module and receiver you are using? If you use something like FASST it might work with newer Futaba so you don't have to buy new receivers for your entire fleet.