open the radiator or coolant overflow tank, start the car and watch for any bubbles. A blown head gasket will often push exhaust gases into the coolant causing bubbles.

Could be a blown intake gasket. Very common on the 3800. Its expensive either way.

check the plastic coolant elbow right above the waterpump. very common for them to leak. there are usually 2 of them. almost all parts stores have them since they leak so often. i just changed mine and also saw the waterpump had a noisy bearing but that just meant i fixed 2 things. the elbows are $5. but a shop might want several hundred for labor.

this design motor routes the coolant thru the alternator mounting bracket using these coolant elbows. you do not have to remove the bracket to change the waterpump but it makes it much easier. there is a small chance whoever changed your waterpupmp may not of removed the bracket. maybe the elbow was good to begin with and the elbow is now cracked and leaking. hard to say. but its a good place to start.