The achievement pops when you unlock a Jack of All Trades Destiny—a hybrid Might, Finesse, & Sorcery Destiny. The first Jack of All Trades Destiny is the Seeker whose requirements are a minimum of two points each in Might, Finesse, and Sorcery. Meeting these requirements unlocks the destiny. It does not have to be equipped to earn the achievement.
You can check your destinies by pressing
and selecting "Destinies." At the top, you will see, "Unlocked Destinies" followed by a number in parentheses indicating how many destinies are available. The locked destinies are shown below. Each entry—Might, Finesse, Sorcery, Finesse/Sorcery, Might/Sorcery, Might/Finesse, and Might/Fin./Sorc. [sic]—is followed by a number in parentheses indicating how many destinies in each category are locked. Before you earn the achievement, the Might/Fin./Sorc. category will show six locked destinies. If you select the category and then choose Seeker, you will see the requirements shown on the top right: Might 2, Finesse 2, and Sorcery 2. One or more of those three requirements will be shown in red indicating you have not met the threshold for the requirement(s).
As soon as you level up and meet the requirement for Seeker, it will move from the unlocked to locked destinies. Since it is new, it will have a star to the left of it. The achievement will pop regardless of whether or not you equip Seeker. The very short video below shows Might at 6, Sorcery at 11, and Finesse leveling up from 1 to 4. The Seeker achievement is unlocked and the achievement pops after a delay of a few seconds: