Every person is different, however, I dive a Pinnacle Seal 3mm Merino in waters down to 68 degrees. I have heard that the really stretchy Elastiprene versions do not have the same warmth (this goes for all brands so if your current 3mm is one of the really stretchy ones that may be a contributing factor). Also, it is not all Pinnacle models that boast the extra warmth. Only the Merino wool lined versions. Anything colder than 68 (I usually don't go below 60deg) I use a merino lined hooded vest under the 3mm Seal. I would think you would be good with a 3mm of any kind, but if you get cold really easily then go for the Pinnacle merino lined versions (non-elastiprene) if you are set on 3mm. This one is the elastiprene and not merino lined.
Elasta Stretch 3MM Womens Wetsuit, Wetsuits: Women, ScubaToys, Elasta Stretch 3MM Womens Wetsuit
Another option to a new wetsuit is a core warmer or hooded vest. Pinnacle makes both in merino lined and I got my 3mm Merino hooded vest for $80.00.