I plan to drain and refill the coolant for 99 malibu V6.
Besides the radiator pet-cock, is there any engine block drain plug?
If yes, how many of them and where is the location?
Any information is highly appreciated. Thanks.

There really is no need to do the block. Drain the Radiator. Fill it up with a gallon of water. Run the engine up to temperature. Drain the water. Do this again, and you can put in the new goo maker fluid, er I mean, coolant.

Agree 100 Percent Beatnik.