Without reading the contract, all we can do is stab in the dark. Unless a different term is specified, I would expect the three year term to start as of the date you left that employment. The term "any activity" is very broad, and can be reasonably said to encompass "any activity".

The larger question is whether a three year term for the contract is reasonable, and that's something you should discuss with a lawyer based upon the full facts and nature of your former employment. Three years is a long time. And "any activity" language is very broad.