What are the optimal percentages for analog system

I disagree with suggestions so far. With the TT you have and the cart, I would DEFINITELY get a better phono section.
The P3 is not a 'entry' table. Plenty of serious vinyl enthusiasts use it. The cart you have is OK
The table is a damn fine basic table, and if you like the sound of Grado, then you have a good combination there.
(you could easily go to a $500 to $700 cartridge later on your Rega P3 and be very happy all over again (if you get a good phono prepre.) Your current phono section is a really basic entry level one, and any used $500. preamp is going to bring your system to a whole new level.
To go for a top TT and cart with that cheap Bellari is foolish IMO.
Anyway, that is my suggestion.
I own:
Rega P5 with Benz Glider, to a Bryston 1.5/BBP-26
and
Kuzma Stogi S with Dynavector 17D3 to Audio Research SP-15 I use just for the tube phono section at this time, to the Bryston BP-26 also.
6,000 LPs.

This is a little complicated especially not knowing your budget and how far in your upgrading you are prepared to go in the near future. But I agree with those who suggest you get the best table you can. Good $200 cartridge will sound OK with $2000 table, $1000 arm and $400/$500 phono stage. With decent cables of course. You could also consider voltage stabilizer/conditioner or/and motor controller. Turntable can last for a very long time.
I personally would probably wait and upgrade everything at once and would spend at least 50% on the table alone.

Where should most of the money go...cartridge, phonostage, or table?

Imo, the priority goes like this: Table, Arm then Phonostage then Cartridge. For now, with your current gear, I agree with Elizabeth. What price range were you thinking?

There are no hard and fast rules but something like this might be a fair starting point:

Table/Arm- 50%
Phono Preamp- 35%
Cartridge- 15%

IMO
YMMV

Thanks for your responses. It seems like I should start with upgrading the phonostage first. I'm looking to spend about $500 on a used unit.