i have audi a3 1.8T 2003 on starting my car taking 3 or 4 crank then it start but engine no power......

It almost sounds like that mechanic is trying to take you for a ride, or your description is not clear. Like what do you mean that the timing is missing? How many miles are on this vehicle? The camshaft sensor is not giving you the wrong information. There is some thing up with the timing, but not so sure it is the timing belt. If there was a problem with the belt, like jumped time. you would have bent some valves, as this is an interference engine. I think I would have someone else look at this.