Do I need a med or long rear mech for 3×9?

Long. Or ‘SGS’ if shimano.

depends

a long cage (shimano or sram) should cover any chainring/casette combo

some mediums might cover certain (closer) chainring/casette combos
eg 42-32-22 with a 12-27T casette

check the tooth capacity of the rear mech. An slx GS 9spd is 35T iirc, whereas an XT 9spd GS is only 33T (not max casette sprocket, difference between small-small and big-big combo)
Not sure about sram

if you never want to use ‘wrong’ combinations like big ring-big sprocket and small ring-small sprocket and set the chain length to suit, then you could use a mech with a smaller tooth capactity

At one point (2008) Specialized were selling at least the ‘WSD’ Stumpjumper FSR with 44-32-22/11-34T chainring/casettes with medium cage (Sram) rear mechs. (website lists so too). So small-small combos meant the mech wasn’t taking the slack

Long, I made the mistake of using a med cage X0 mech with 3 x 9 front.

Main problems were the bottom jockey wheel wears quicker and you can trap mech when in top back and front.

Does however work well on 27-39 front and 11-32 rear.

Always used a medium cage without issue.
Med is fine as long as you’re disciplined with your shifting.
If you like to run large/large go with a long (or get a double set up!)

Think I’ll go with the long just to be on the safe side. Thanks.

I have two race bikes, both with 24,32,44 rings. The full suss has a short XTR Shadow the HT a long.
My wheels have 12-27 (Shimano DA) and 11-32 (SRAM PG990) cassettes and I swap them around all the time, various combinations of full-suss/HT, strong/light wheels, big/little cassettes.
The size of the mech has never been a problem and all combinations have worked perfectly (9sp btw).