Combine the cream cheese, sour cream, garlic powder, lemon zest, salt, and dill until smooth and well mixed. Spread half of this mixture over one flatbread, shaping it into a neat square. Lay half of the smoked salmon atop the spread. Thinly butter a second flatbread, then position it butter side down over the salmon. Spread the remaining filling over the top wrap, again forming a square, then layer with the remaining smoked salmon. Lightly butter the third wrap and press it, buttered side down, onto the salmon. Using a flat object such as a cutting board, press down gently to compress and even the layers. Wrap the entire stack tightly with cling film and refrigerate for at least 4 hours, or up to 48 hours, to allow the filling to set firmly. When ready to serve, trim the edges with a serrated knife using a gentle sawing motion to create a clean square. Slice the square into 6 or 7 even strips, then divide each strip into bite-sized squares. Present on a platter with toothpicks, accompanied by lemon wedges and extra dill for garnish if desired.