You need to fill the radiator completely with coolant, making sure you get all the air out. When you fill the reservoir, nothing will enter the radiator - it's still empty, that's why you didn't see any coolant leaking out when you restarted it. Fill the radiator, let it get warm, then look for leaks.
The 3.8L V6 has a common problem with some fragile "elbows" that sit on or near the serpentine belt tensioner. I see these mentioned all the time on another site. Apparently there are some metal replacements available. Might want to check that area for the leak.
94 Formula A3+1: 381ci all-forged stroker - Callies Stealth, Oliver 5.85 billet rods, BME nitrous pistons / CNC LT4 heads / Comp Cams custom solid roller / TH400+GearVendors OD/4.11's / Strange 12-bolt / 300-shot N2O / Spohn Suspension / 6-pt roll bar with integrated SFC's / full AutoMeter instrumentation / MoTeC M48 Pro engine management system / a few other odds 'n ends. 800HP/800LB-FT at the flywheel.