Toss the strawberries, raspberries, and blueberries with 0.25 cup granulated sugar and the lemon juice in a large bowl; let macerate until glossy and syrupy, 15β30 minutes. Reserve about 1 cup of the prettiest berries for topping and keep the rest for layering.
Heat the whole milk and kosher salt in a medium saucepan over medium heat until steaming and small bubbles form at the edges, 3β5 minutes.
In a separate bowl, whisk the egg yolks, cornstarch, and the remaining granulated sugar until pale and thick, 1β2 minutes. Slowly whisk in the hot milk to temper, then return the mixture to the saucepan.
Cook over medium heat, whisking constantly, until the custard thickens and bubbles break the surface; continue whisking 1β2 minutes more to cook out the starch. Remove from heat and whisk in 2 teaspoons vanilla extract and the unsalted butter; reserve the remaining 1 teaspoon vanilla extract for the whipped cream. Spread the custard in a shallow dish, press plastic wrap directly on the surface, and cool to room temperature (30β45 minutes) or set over an ice bath to speed cooling.
In a chilled bowl, whip the heavy cream with the powdered sugar and the reserved vanilla to soft peaks, 2β3 minutes. Cover and refrigerate.
Gently warm the raspberry jam in a small saucepan over low heat or in the microwave until spreadable, 15β30 seconds; cool slightly.
Arrange half of the ladyfingers snugly in the bottom of a 3-quart trifle bowl. Drizzle evenly with 0.25 cup medium-dry sherry. Spread with half the jam. Spoon over half of the macerated berries (leaving excess juices behind), then half the cooled custard, smoothing to the edges.
Repeat with the remaining ladyfingers, 0.25 cup sherry, jam, berries, and custard. Press plastic wrap directly on the custard surface.
Chill until the layers meld and the biscuits soften, at least 4 hours and up to 24 hours.
Uncover, mound the whipped cream on top, and swirl. Garnish with the reserved berries and fresh mint leaves. Spoon into bowls and serve.