Does my fiancée's ex wife have rights to spousal support after 3 years? And once we get married will I have to pay as well?

No, you don't have to pay his spouse. You both need to educate yourselves.

As long as your fiancée is gainfully employed comparably to her past employment the Court should have no basis for looking past her earnings to yours for determining your fiancée's X's support.

Ordinarily, remarriage of the "obligor" spouse in and of itself is not considered a "substantial change of circumstances" that would give rise to a modification of spousal support. However, if there were other substantial changes from when the judgment was entered the new spouses contributions that reduce h living expenses can be considered as one of the factors of ability to pay, just as all those people who live with her and contribute to her living expenses would impact on her ongoing need. This is a very complicated area of the law, which would require a weighing of all of the factors set out in California Family Code section 4320, so if she does file a request for spousal support you should hire an attorney who regularly practices family law to represent your fiance.