\(MCD:\left(R3ntR2\right)//R1\)
\(\Rightarrow R=\dfrac{R1\cdot\left(R2+R3\right)}{R1+R2+R3}=\dfrac{20\cdot20}{20+10+10}=10\Omega\)
\(U=U1=U23=12V\)
\(\Rightarrow I=U:R=12:10=1,2A\)
\(\Rightarrow I1=U1:R1=12:20=0,6A\)
\(\Rightarrow I23=I2=I3=U23:R23=12:20=0,6A\)