Why did Yaakov see fit to relate everything that the angel told him to Rachel and Leah?
Ramban (to 31:13): Because he felt that this would convince them to accompany him when he left Charan. 1
Ha'amek Davar (to 31:10): They should not think that the property belongs to their father, and he will get it back and give it to them as Nichsei Melug.
Ha'amek Davar (to 31:13): They should know that they must leave immediately, for if not, Hashem's promise to guard him from Esav will not apply. Also, he must not delay fulfilling his vow. If a man delays, his wife is prone to die! 2
Ramban: As a matter of fact, the two current Pesukim are referring to two different dreams. 31:12 goes back to when Yaakov was still tending to Lavan's sheep (during the course of the last six years), whereas 31:13 is referring to the end of that period. And on the very next day, he left Charan.
Zevachim 29b, according to Tosfos. (Even according to Tosfos, she dies only if he does not fulfill, but not for delaying! Perhaps it means that she will not die after he fulfills, but before he fulfills, she is prone to die. However, Tanchuma 35:1 explicitly says that Rachel died (after he fulfilled it) due to his delay! - PF)
Why were there three appearances? They should all be the appearance that Yaakov receives!
Ohr ha'Chayim: Lavan had Terafim that told him everything, including what is in the womb. Hashem is reluctant to do a great miracle to change it. Rather, he made each fetus latent with the three appearances, so whatever they will agree on, it can come out for Yaakov. There are birds that can change their color! Malbim - the angel told Yaakov to return, for Hashem does not want to constantly do such miracles and Hashgachah.