Do your Abs and Arg functions not return an output or do they not return the result you are expecting?
A couple of pointers. It may be a result of the formatting of your post (it is best to use the code formatting tools above the posting area to show code) or you may be actually missing some spaces. So the following will be interpreted by Mathematica as a single parameter, not a product of I, w, and C3:
IwC3
This should be written with spaces to imply multiplication as
I w C3
With that said, here are your input expressions:
Z1 = 1/(I w C1);
Z2 = 1/(I w C2) ;
Z3 = 1/(I w C3) ;
Z4 = ((R1 + Z1)* Z2/(R1 + Z1 + Z2) ) ;
Z5 = Z4 + R2 ;
Zth = Simplify[Z5*Z3/(Z5 + Z3)] ;
Where I put semicolons at the end of each to suppress the output. To get a result from evaluating Abs[Zth] that actually gives you the absolute value, you need to process the result along with telling Mathematica that w, C1, C2, C3, R1, and R2 are real. You can do this with ComplexExpand which assumes that all variables are real unless you tell it otherwise:
ComplexExpand[Abs[Zth]]
Which gives
$ \frac{\sqrt{\left(1-\text{C1} \text{C2} \text{R1} \text{R2} w^2\right)^2+(\text{C1} w
(-\text{R1}-\text{R2})-\text{C2} \text{R2} w)^2}}{\sqrt{w^2} \sqrt{\left(\text{C1}
\left(\text{C2} \text{C3} \text{R1} \text{R2}
w^2-1\right)-\text{C2}-\text{C3}\right)^2+(\text{C1} (-\text{C2} \text{R1} w-\text{C3}
w (\text{R1}+\text{R2}))-\text{C2} \text{C3} \text{R2} w)^2}} $
as the result. To compute Arg[Zth] you can again use ComplexExpand, but you need to tell it what target functions to do the expansion with respect to as in (Also using FullSimplify with assumptions that the parameters are Real to simplify things a bit)
FullSimplify[ ComplexExpand[ Arg[Zth], TargetFunctions -> {Re, Im}], {{w, C1, C2, C3, R1, R2} \[Element] Reals}]
Yielding
$\tan ^{-1}\left(\frac{\text{C1}^2 \left(\text{C2}^2 \text{R1}^2 \text{R2}
w^2+\text{R1}+\text{R2}\right)+2 \text{C1} \text{C2} \text{R2}+\text{C2}^2
\text{R2}}{\text{C1}^2 \text{C2}^2 \text{C3}^2 \text{R1}^2 \text{R2}^2 w^4+w^2
\left(\text{C1}^2 \text{R1}^2 (\text{C2}+\text{C3})^2+2 \text{C1}^2 \text{C3}^2
\text{R1} \text{R2}+\text{C3}^2 \text{R2}^2
(\text{C1}+\text{C2})^2\right)+(\text{C1}+\text{C2}+\text{C3})^2},-\frac{\text{C1}^2
\text{C2}^2 \text{C3} \text{R1}^2 \text{R2}^2 w^4+w^2 \left(\text{C1}^2 \text{R1}^2
(\text{C2}+\text{C3})+2 \text{C1}^2 \text{C3} \text{R1} \text{R2}+\text{C3}
\text{R2}^2 (\text{C1}+\text{C2})^2\right)+\text{C1}+\text{C2}+\text{C3}}{\text{C1}^2
\text{C2}^2 \text{C3}^2 \text{R1}^2 \text{R2}^2 w^5+w^3 \left(\text{C1}^2 \text{R1}^2
(\text{C2}+\text{C3})^2+2 \text{C1}^2 \text{C3}^2 \text{R1} \text{R2}+\text{C3}^2
\text{R2}^2 (\text{C1}+\text{C2})^2\right)+w (\text{C1}+\text{C2}+\text{C3})^2}\right) $
And here is an alternative approach with a different TargetFunctions option:
FullSimplify[
ComplexExpand[ Arg[Zth],
TargetFunctions -> Conjugate], {{w, C1, C2, C3, R1, R2} \[Element]
Reals}]
giving
$-\frac{1}{2} i \left(\log \left(\text{C1}^2 \text{C2}^2 \text{C3}^2 \text{R1}^2
\text{R2}^2 w^4+w^2 \left(\text{C1}^2 \text{R1}^2 (\text{C2}+\text{C3})^2+2
\text{C1}^2 \text{C3}^2 \text{R1} \text{R2}+\text{C3}^2 \text{R2}^2
(\text{C1}+\text{C2})^2\right)+(\text{C1}+\text{C2}+\text{C3})^2\right)-\log
\left(\frac{w^2 \left(\text{C1}^2 \left(\text{C2}^2 \text{R1}^2 \text{R2}^2
w^2+(\text{R1}+\text{R2})^2\right)+2 \text{C1} \text{C2} \text{R2}^2+\text{C2}^2
\text{R2}^2\right)+1}{w^2}\right)+2 \log \left(\frac{-i \text{C1} \text{C2} \text{R1}
\text{R2} w^2-w (\text{C1} (\text{R1}+\text{R2})+\text{C2} \text{R2})+i}{\text{C1}
\text{C2} \text{C3} \text{R1} \text{R2} w^3-i w^2 (\text{C1} \text{R1}
(\text{C2}+\text{C3})+\text{C3} \text{R2} (\text{C1}+\text{C2}))-w
(\text{C1}+\text{C2}+\text{C3})}\right)\right)$
So this should give you a sense of approaches you might take...
