Have a vt ss changed heater valve,radiator,welsh plugs and thermo. car is still loosing water and overheating why?

Check the cooling fans, you may need to by a non contact thermo temp gun to verify that the car is really over heating. the are available to harbor freight.

no
loosing water is a concern but the correct method before adding anything to the cooling system is to find the leak
A head gasket can be checked by a compression test
leaks can be determined by a cooling system pressure test
The leak can be from a rusted out welsh plug, water pump seal , radiator cor hole , heater core failure

Try buying a manual.
http://onlyrepairmanuals.com/holden/holden-commodore-vt-vx-vy-vu/
Using the manual above wWhen I did my welsh plugs last year it was easy as. I got it free.. i think it's paid for now.

if its not overheating your water is circulating and working normally
your heater core or rad could be airlocked open a top heater hose and fill slowly making sure no air in system run for a while then top up can take a while to clear the air
also make sure heater valve is operating

Your wasting your time and money doing that lot as your problem is either the heater hose on L/H/Side of engine split or leaking tap for heater or head gasket or the REAR CORE PLUG in rear of engine block rusty and needs replacing (dead centre of rear of block) but you will need to either use a mirror to see it or remove the engine and replace ALL welsh plugs.
Let me know how you get on.
Ron

A couple of things to check because as you describe your symptoms you actually have 2 problems

1. No heat from vents.

Sounds like the heater water flow valve cable is kinked disconnected or the valve itself is stuck You will need to follow your heater core water supply lines to find the valve usually to the right of the center console in the ducting area / large plastic housing for heater and AC internal coils. If the valve works then there is a possibility that the heater core water supply lines have been bypassed on the engine side of the firewall with a coupling due to a leaking heater core. And l;astly the heater core could just be clogged. (quite rare!!)

As for the overheating engine after all of the work you have done.. Perform this simple test.

1. remove radiator cap
2. have someone else start the car
3. watch for water coming out of the open radiator cap area DURING start...
4. have person in the car rev engine gently 2 or 3 times. No need to exceed 2000 RPM. If water rapidly flows out of the radiator during engine revs.. you have some major engine issues.
If it passes all of these checks then the only thing left is improper hose connections on the engine side of the firewall and or a hydraulic water lock in the block that should clear itself by starting and stopping the engine many times with the radiator cap off.

Is there a bleeder valve on the thermastat housing or the water pump. if not check to see if your loosing coolant and the oil level

i would change the water pump and check for plugs in the heater core

i doubt very much wether the block is cracked it is more likely to either be a heater hose may be leaking or a welsh plugs ( little round dish shaped plug ) in the side of the block i would first check all the heater hoses and the look on the side of the motor for a possible welsh plug leak i'd say its hoses tho

Possibly a bad hose to the heater core or the heater core could be plugged,especially since you describe getting no warm air from the heater and coolant flowing between motor and transmission,the cooling fan could be a bad thermo switch or relay,as far as the freeze plugs most likely not,the coolant would flow out of a bad freeze plug as soon as you started filling it,you should go back to your "mechanic" and demand a refund,I'd say demand a repair but I don't think this guy can fix is own hair,it sounds like he was hoping and groping for a solution,at the very least file a consumer complaint with your states attorney general and cosider a small claims suit.