The answer from Al Berko doesn't give actual sources for his answer and his explanation is difficult, at best to follow. But looking at the original question points to the answer to your question.
The Ohr HaChaim to Bereshit 29:18 comments on Yaacov offering to work 7 years for Rochel that a Tzaddik always subjugates himself seven. (עצמו שבע) This relates linguistically to the number seven, the verb to satisfy, and to making an oath. He says it relates to the Sod concept from Mishlei 24:16, שבע יפול צדיק וקם. That through subjugating for seven the adversary is satisfied and overturned by HaShem.
This same idea is mentioned in Ba'al HaTurim to Bereshit 46:2. There, the Tur mentions that when writing the word לישראל in a sefer Torah, seven crowns are placed upon the letter Shin as a sign that G-d would save Yaacov from 7 troubles.
The letter Shin has 3 arms and also has a Mispar Katan value of 3.
It is unclear what Al Berko is referring to in regard to the number 3 except that it may be a reference to the concept of chazaka. That repeating something three times establishes it as a fixed condition.
He points to the idea that Yaacov was 84 years old when he actually married Leah and Rochel. When divided by 7 it results in 12, which could be an allusion to his comment that Yaacov stole the blessing of his brother, Eisav.
According to Berko in the name of the Ari z"l and the Ba'al Shem Tov, that included that Eisav would have been the father of the twelve tribes of Israel. Specific sources for those two references were not provided.
And if 84 is divided by 21 (3 times 7), the result is 4 which could be a reference to G-d's four letter name, who saves Yaacov from his seven troubles.