As above. Yes better to upgrade the turntable, rather than a cartridge upgrade, as, and along with the wise comments above, unlikely to hear the dynavector at its best on the Planar 3. If you heard it on a higher spec turntable/arm combination, I think you’d be surprised that you were listening to the same cartridge.
If I had a P3, and was generally happy with the sound, i’d Probably stick with a Rega cartridge, but maybe their top moving magnet, as a price/synergistic match.
Dealer I know, also concurs that the P6 is the sweet spot in the range, and thinks you can bypass the P8, and if you still aspire to high end Rega tt’s, then go to the P10, if within budget, and better still, a mid spec LP12, if you can afford it. Different league, but also different price.