I've heard this as "Three disasters and an ending" or "three falls and a victory". It's not a firm rule of any kind, but if it works, it's not a bad plan.

In less rigid terms, it's best to not let the protagonist win all the time, and it's often the case that he/she only wins at the last showdown. Or, at the very least, that all victories before the last one will have flaws.

...
Regards,
Eric Fretheim
Assistant Prep Leader, 2015 October NaNoWriMo Prep Challenge

"It is perfectly okay to write garbage-- as long as you edit brilliantly." ~C.J. Cherryh

“No, writing 50,000 words in a month is normal. You are not crazy. This is not insane.” ~Teri Brown