I think some thing like the OPV
Built to merchant standards about 1500 - 1700 Tons.
2 Diesel Engines driving 2 props - max speed 20 knots - 4000 plus NM range
Large helipad to the rear (no hanger or facilities) Chinook/Sea stallion capable that can double up as storage area or mission specific containers
Weapons -
OTO 76mm compact automatic gun system (or even a Bofors 40/60 if the budget is tight?)
2 (1 each Port and Starboard) Wing mountings for .50 cal machine guns
2 additional mountings on port and starboard Quarter for GPMGs
2 large RIBS - deployable 1 each Port and Starboard
Troop capacity of up to 60 men
Troop Space can be converted to a operational command post or medical center or evacuating civvies, Children's party's and Sods Operas
But a lot depends on the mission but this design should be good for the 3 block war scenario the OP speaks of