When the three angels visit Avraham, the mefarshim go to length on why each Malach was needed and the different missions each was required to carry out - why don't we have the same when 5 malachim visited Hagar in the previous parsha and what were these 5 unique missions that required 5 malachim?
It appears that you are referencing Lech Lecha 16:9 - 12 since Raashi says
And the angel of the Lord said to her, etc.: For each statement, another angel was sent to her. Therefore, the word מַלְאָךְ, angel, is used with each statement. — [from Gen. Rabbah 45:7]
There are meforshim such as Rav Sorotzkin who explain that Hagar refused to listen to the malachim and in effect sent them back with the "mission" a failure. As a result, each message, being a different promise, required a different mal'ach.
- Go back to Sarai and bear this child who will be part of the family of Avram and Sarai.
She refused and had a miscarriage.
- Go back and you will have children who will become great.
This is similar to what happened with her (as Keturah) after the death of Sarah when Yitzchak brought her back to Avrohom.
She again refused and the mal'ach had to end his mission leaving her behind.
- You will get pregnant and have Yishmael who will be a wild man (פֶּרֶא אָדָם) and terrorize to world.
This is what she finally accepted and when she agreed to go back. This is analogous to the modern Arab mothers who raise their children to become terrorists.
Thus, each time it says וַיֹּאמֶר לָהּ מַלְאַךְ יְהֹוָה it was actually a new mission.