When the player selects the Elf dialog, instead of setting their race, create a new dialog. In this dialog, have a choice of the 3 elf races that when clicked, create those starting units.
E.g When a player selects a dialog other than Elf, create their starting units of the selected race
If the selected race is Elf, create a second dialog listing the 3 choices, and when one of those is clicked, create those starting units.