I have a 04 Audi A4 my car over heats and leaks coolant I had the hose thermostat and bottle changed now it won't start I also have a low compression in cylinder 3 which misfires is it the head gasket

That car has a timing belt. Has it ever been replaced?

No

Ooo, then it needs to be. But I am not saying that is the problem. If the coolant is leaking where it can be seen, like puddles on the ground it needs a pressure test to see where the leak is. But if you mean it's leaking out where you don't know where it's going, maybe out tail pipe, could be head gasket. But if the timing belt jumped a tooth it may have bent a valve, and that is the compression loss and could be from

Do u kno the estimate to fix the head gasket

Somewhere in the $1300 to $1500 range. That includes an oil change and coolant change.. not just topping off what was lost during the job. I went to two different auto repair estimate sites and they agree on that figure. It also includes sending the head out for a valve job and re-surfacing, and a X-ray to check for cracks. And just a friendly warning..this is an interference engine and if your timing belt breaks you will need a rebuilt engine. It is 6 years overdue

Would I be wasting my time putting money into this car

KBB says it's worth $4600. I went with 'Fair' condition and not knowing mileage I just plugged in 75,000. So that's a close call. If it has a lot more than 75k miles, or is in not-so-good condition, then I'd say yes, it would not be wise to throw that much money in to a $3000 car. On the other hand, if the car is in really good shape, no body damage, less mileage than that, then that changes things.

With timing belt, and head gasket (if that really is blown) then you will spend more like $2k

Daaam....ok thanks alot

Yea has 148k miles with a dent on rear side

This car may be misfiring due to overheating after driving without coolant.