rovnice={
iU[t]+iR2[t]==iR1[t]+iL[t],
0==iR1[t]+iL[t]+iC1[t],
iC1[t]+iR2[t]==iC2[t],
u2[t]-u1[t]==R1*iR1[t],
u1[t]-u3[t]==R2*iR2[t],
iC1[t]==C1*(u2'[t]-u3'[t]),
iC2[t]==C2*u3'[t],
u2[t]-u1[t]==L*iL'[t],
u[t]==u1[t],
iL[0]==0,
u2[0]==0,
u3[0]==0
};

nezname={u1[t],u2[t],u3[t],iU[t],iR1[t],iR2[t],iC1[t],iC2[t], iL[t]};
soucastky={R1->3300, R2->47000,C1->0.000000039,C2->0.000000082, L->0.022};
(*soucastky={R1->1, R2->1,C1->1,C2->1, L->1};*)
amplituda=5;
frekvence = 1000;
(*u[t_]:=1*)
u[t_]:=amplituda*(1+Tanh[100 Sin[2*Pi*frekvence*t]])/2
reseni=NDSolve[rovnice/.soucastky, nezname, {t, 0, 0.003}, StartingStepSize->(10^(-9))];

Plot[{u[t],u3[t]/.reseni}, {t, 0, 0.003}, PlotRange->All]