The running Locust is probably from the 3rd Edition boxed set. One of the legs is on a peg, so it could be positioned on the ground or up in the air in a running pose. If there's an LCT engraved in the front hexbase facing, that's the 3rd Edition Locust.
The other Locust and the Catapult sound like they're from the PlasTech box, which came with two each of Catapult (Fatapult), Atlas (Fatlas), Blackjack, Locust, Hunchback, Trebuchet and Valkyrie. That Locust was in the pose you described and, while it did have a chin turret, it was very small and stubby compared to the 3rd Edition or metal version. I'm not sure about the Catapult having a RT gun, but the Fatapult from PlasTech did have some blobby medium lasers on either side of the torso.