You can fire everything if you remain stationary. The rule says:
A Leman Russ that moved at combat speed or remained stationary can fire its turret weapon in addition to any other weapons it is usually allowed to fire (even if the turret weapon is ordnance!).
In all cases, you say, "I'm going to fire as many weapons as I can fire given how far I moved" plus "I'm going to fire my turret, using the Lumbering Behemoth rule."
If you moved at combat speed, that's one weapon plus turret. But if you remained stationary, that's all non-ordnance weapons plus turret (ordnance or not). So you can absolutely fire a hull-mounted lascannon and two sponson-mounted heavy bolters (all three of which are non-ordnance, non-turret weapons) plus a turret weapon (even if it is ordnance).