A LA TERCERA FUE LA VENCIDA= In third was the charm.- LA TERCERA ES LA VENCIDA= third time is a charm (PROZ) / Three strikes and you're out / the third time 's the charm / third time lucky (BrE).

That's actually a good question that made me think :D I would say this phrase comes from "La tercera (vez) es la vencida" (literarily "the third time it's the overcome one"), or "La tercera vez será la vez que vamos a vencer", which doesn't sound very good like that, but could be thought as "En el tercer intento vamos a superar el problema" = "At the third attempt, we'll overcome the problem". That's what I can think of. This is one of many phrases that we use without even thinking if they make sense.

what do mi ex y yo dandonos nuestras trigesima oportunidad por que la 30 es la vencida mean

The literal translation for me would be : "the third one is the overcomed one"

THE EXPRESSION DOES NOT HAVE LITERAL TRANSLATION, IT IS WRONG TO SAY : the third is the beaten/victory/vanquished OR the third is the time you will make/achieve/reach/overcome it ( EXPLAIN THE EXPRESSION IS NOT LITERALISM). it does not have literal translation. THE LITERAL TRANSLATION IS NOT POSSIBLE BECAUSE OF THE METAPHOR. third time lucky= suerte a la tercera vez OR la tercera es la afortunada IS WRONG, third time lucky= la tercera es la de la suerte OR la tercera es la vencida METHAPHORICALLY SPEAKING IS RIGHT.