Where is the low side charge port on a 96 Regal with 3800 engine?

A 1996 Buick Regal with a 3800 engine doesn't have a low-side charge port in the way a modern vehicle with air conditioning refrigerant would. Older cars like that used a different system for refrigerant charging. There's no single "low-side" port to connect a recharge hose to. A qualified mechanic would use a manifold gauge set and access the system through the various service ports on the A/C components themselves.