It was ordained that the number of daily services should equal the number of daily sacrifices. The two daily services parallel the two daily sacrifices. When every day when a special additional sacrifice was offered, a third service was ordained, paralleling this additional sacrifice.
The prayer that parallels the morning sacrifice is called Shacharit. That which parallels the afternoon sacrifices called Minchah. The service that parallels the additional sacrifice on the Shabbat and Festivals is called Mussaf.
It was also ordained that each person should offer a prayer at night. This is because the afternoon sacrifice would be allowed to burn on all night on the altar. It is thus written in this week’s portion, “This is the burnt offering on the embers all night long until morning, and the fire of the altar shall be kindled with it (Leviticus 6:2).”
This evening prayer is not an obligation like the morning and afternoon services. Nevertheless, all Israel, wherever they live, have accepted the custom of reciting the evening prayer. It was therefore accepted just like an obligation. (Rambam; Yad haChazakah, Hilchot Tefillah 1:5-6)