R1=5.*(10^6);
c1=500*(10^(-12));
R2=1*(10^6);
c2=2500*(10^(-12));
pomerZadany=100;
ser:=Plus@@#&;
par:=(((Plus@@(((#)^(-1)))))^(-1))&;
ω=2*Pi*f;
z1=par[{R1,(1/(I*ω*c1))}];
z2=par[{R2,(1/(I*ω*c2))}];
z3=par[{R3,(1/(I*ω*c3))}];
prenos=(z3/(z1+z2+z3));
odchylka[f_]=Abs[prenos-(1/pomerZadany)];
startR3=R3/.Solve[(R3/(R1+R2+R3))==(1/pomerZadany)][[1]];
fmin=1;
fmax=1*(10^6);
q=1.2;
nf=Round[((Log[(fmax/fmin)])/(Log[q]))];
frekvence=fmin*(q^(#))&/@Range[nf];
promin=Plus@@(odchylka/@frekvence);
resmin=FindMinimum[promin,{{R3,startR3},{c3,5*(10^(-9))}}]
LogLogPlot[{Abs[prenos/.resmin[[2]]],(1/pomerZadany)},{f,fmin,fmax},PlotRange->All,AxesOrigin->{fmin,(0.1/pomerZadany)}]