“Policemen aren’t supposed to believe in coincidences,” we’re told in the third series of Wallander, and mystery fans wouldn’t have it any other way. The appeal is in the slow piecing of details, and the solutions need to feel earned. Henning Mankell’s burdened Swedish detective perseveres in case after case, driven to explain the inexplicable horrors that are visited on the bleak Scandinavian countrysides. Kenneth Branagh balances the intensity and weariness of lead inspector Kurt Wallander, and he is supported by equally strong yet understated performances. The newest BBC release includes beautifully directed, feature-length adaptations of “An Event in Autumn,” The Dogs of Riga, and Before the Frost. You won’t be disappointed.