The trio will be joining previously announced stars Davi Jay, Barry Tubb and Roark Critchlow, and will be shooting in Austin, reports Deadline.
Dear Sidewalk, directed by Jake Oelman and written by Jake Limbert, centres on "a young mail carrier who falls for a recently relocated divorcee on his route".
Former child actor Mazzello previously appeared in the HBO miniseries The Pacific about the US Marine Corps' involvement in the Pacific Theater of Operations during World War II.
Forbes is best known for her roles in series dramas such as Guiding Light and HBO's True Blood, where she played the supernatural creature Maryann Forrester.
She can be regularly seen in AMC's The Killing, which is currently airing its second season on Sundays at 9pm ET.
Watch a promo for the next episode of The Killing below (US only):