have to replace the fuel, fuel return and the vent. Can't find a diagram that shows any routing or pictures of the fuel tank assembly.  TIA teo

I only see one line going to the carberator per the parts diagram. 
 
That line is the intake fuel line going from the in tank filter to the only carb nipple.
 
The vent, see below pic:. 
 
Are you certain you have a third line?  
I don't see a third line. 
 
Part # Husqvarna 503600401
Filter Plug is the venting filter
 
 

I only see one line going to the carberator per the parts diagram. 
 
That line is the intake fuel line going from the in tank filter to the only carb nipple.
 
The vent, sits in the plastic housing. 
 
Are you certain you have a third line?  
I don't see a third line.