As far as I know, Tikka only makes the 4 round version in 6.5 PRC, and that's what I'd expect the rifle to ship with.
https://www.eurooptic.com/tikka-t3x-65-prc-4rd-magazine-s540210292
If you want a flush-fit 3 round version, the L mag will work fine and not limit you on length. I have ran several hundred rounds through my 6.5 PRC with each of those two types of mags without issues. I'm hand-loading mine with a COAL of 2.980 and they fit in the PRC magazines with a good amount of clearance.
I'm not sure which bolt stop they ship with, but you could run into an issue there with L magazines if it doesn't have the long action bolt stop.