Which upgrades for my Rega Planar 3?

The Rega 3 is a very under-rated deck. I've had mine for over 25 years and it has out-performed Linns and Sota Cosmos regularly.
The trick is really the arm that you use.....I've used a 30 year old Hadcock GH 228 unipivot which the Graham and VPI unipivots studied very well when in their design stages.
Also what helps, is making a thin platform out of 12mm laminated MDF with 3 adjustable screws positioned under the 3 rubber feet. This allows for EXACT levelling of the deck and rejects the absorption of air-borne low-frequency feed- back through the base.
The Rega arm is NO contest for the Hadcock and you ain't heard the Rega 3 until you hear hear it with a great unipivot like the Hadcock.

Look into the Graham ones, by the way what is your budget.

Boston Audio Mat1 platter mat - $199, Mapleshade Heavyfoot cones x 3 - $120 into a Mapleshade Maple Platform - $75, suspened on sorbothane squares - $20, Kerry Audio Design F2 counterweight - $129 and Acrotec armwire straight thru to male RCA - $150.

Well there you go ,take your pick....or picks.

I upgraded my son's Rega with these sorbothane feet, and he describes the sound as "more detailed"   https://www.mnpctech.com/products/rega-turntable-sorbothane-isolation-feet-set-of-three