The Tactical Marine Squad is a heavy infantry unit for Space Marines in Dawn of War III. They can be upgraded to carry flame or plasma weapons, though they can overheat. These are especially useful against other heavy infantry units.
Compared to Dawn of War I the use of Heavy Bolters and anti-vehicle weapons has been separated out into other units, the Devastator squads. Upgrading them with a Flamer gives one unit a toggled Flamethrower ability which does Normal damage in a cone with a moderate cooldown, whereas the Plasma Gun upgrade gives the entire squad Plasma Guns which does enhanced normal damage to all targets but builds up Heat over time, and periodically must stop firing to cool down. Only one of the two upgrades can be chosen.
They are general bread-and-butter fighters, reasonably decent at melee and ranged combat (slightly better at range) but don't excel at either.