In a small bowl, stir together mayonnaise, chives, dill, parsley, lemon zest and juice, salt, and pepper.
Lay cucumber slices on paper towels to absorb moisture.
Trim and discard crusts from bread slices to make even squares.
Spread a layer of mayonnaise mixture onto each bread slice. On 6 bread slices, place 6 to 9 cucumber slices in a single layer without overlapping. Top with remaining bread slices, mayonnaise side down.
Using a serrated bread knife in a gentle sawing motion, cut sandwiches diagonally into 4 equal triangles. Serve immediately, or cover with damp paper towels, place in a covered container, and refrigerate for a few hours until serving time.
Recipe by TeaTime Magazine at https://teatimemagazine.com/vicki-delany-murder-spills-tea/