The wording is very confusing. Could be helped quite a bit, by correcting the tense, at least. Currently, it's...
"The Third Time you gain an Action this turn,. . . [other] players gain a curse."
But Cauldron, of course, is a Treasure, and this means that it is played *after* the action phase has ended. The way the card is worded, it seems to suggest that it does not matter at all how many action cards you were able to gain during your action phase (Remodel, Transmogrify, Expand, etc etc) . . . to give out a curse, you must gain 3 (more?) actions after playing this Cauldron.
Maybe change the language to: "If 3+ action cards were gained on this turn, each other player gains 1 curse for each Cauldron you played this turn."? There is no reason, as far as I can tell, to use "action" rather than "action cards" here, to avoid at least this bit of confusion.
When this card came up in a game with friends, we were very confused. Several of us thought the card was saying, "If you got 3 extra actions this turn (via playing villages), then--during Buy--curses would be given out if Cauldron was played." Obviously, this turned out not to be the correct interpretation.