ARM - LVL3 vehicles
AVATAR LRPC tank - Long range artillery -
Aegis - Mobile Tactical Nuke -
Tiger Super Tank - Multi Weapon HVY assault -
http://www.unituniverse.com/?p=u&v=900
BigWeelle - Mech Slayer/mobile dual ANNI -
http://www.unituniverse.com/?p=u&v=1929
Rhino - Rapid Fire Anti-swarm-plasma
http://www.unituniverse.com/?p=u&v=578
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CORE - LVL3 vehicles
Mauler LRPC tank - Long Range Artillery -
Blitz Salvo Truck - Catapult Equivalent -
http://www.unituniverse.com/?p=u&v=1327
REX - Multi Weapon HVY assault -
http://www.unituniverse.com/?p=u&v=1141
Defiler - All Terrain HVY Assault -
http://www.unituniverse.com/?p=u&v=3357
Opressor - Rapid Fire anti-Swarm-laser -
http://www.unituniverse.com/?p=u&v=1859
These are the best looking and IMO most appropriate heavy Tanks I found on the UnitUniverse Library.
There are still some there that I didn't list, the M4a1 Campbell, and some other very nice Heavy tanks as well.
Keep in mind that LVL3 tanks don't need to all be KROGOTH class tanks, just like the Mechlab includes a whole range of units from small to the largest units.
If any of these tanks that I posted would be concidered for entry into the LVL3 factory, I'm sure that most will require some weapon adjustments to fit into the ARM/CORE style of units.
Perhaps even a Shielded tank would add some depth and range into one of the units.
Also Keep in mind that SLOPE TOLERANCES need to be increased for these larger Tanks so that they can move around dispite their massive sizes without getting easily stuck.
All these tanks are listed here at http://www.unituniverse.com/