EDIT: well I tried a few quick things in Geronimoo's Sim and in a 3-player game, one Workshop/Gardens vs two Smithy-BigMoneys wins ~75% of the time, even if the Smithy players avoid Estates. And then two Workshop/Gardens vs one Smithy-BigMoney wins almost none of the time. Of course the simulator isn't perfect, but it does suggest Workshop-Gardens is hard to beat if only one player plays it.

I played around with this a bit more in Geronimoo's sim, and got some interesting results:

So, as noted, for the baseline if you just compare big money smithy vs workshop/gardens you get:

2 players go Smithy 1 workshop-gardens, then workshop-gardens wins 75% of the time.

2 players go workshop-gardens 1 smithy then smithy wins 100% of the time.

But, interestingly, if you make the workshop-gardens players get Gardens after 2 workshops, then 2 players going for this strategy split the wins 45% each. (Smithy winning ~2% of the time, with 7% ties).

I believe this is because the game ends quicker with this strategy (looks like it goes about 12 turns), which doesn't give smithy-BM time to get more than about 2-3 provinces, but the workshop players get ~30 cards in their deck.

This might not be the optimal mirror strategy, but getting more workshops does worse, and trying for duchies appears to do worse, so it seems to be at least a little bit stable.

Anyways, it looks like based on that test, 3-player workshop-gardens is not a "prisoner's dilemma", but just a winning strategy the majority of the time, if you go after Gardens strong enough.