$${R_1 \parallel R_2 = \frac{2}{3}{}{\Omega},\quad R_1 \parallel R_3 = \frac{3}{4}{}{\Omega},\quad R_2 \parallel R_3 = \frac{6}{5}{}{\Omega},\quad R_1 \parallel R_2 \parallel R_3 = \frac{6}{11}{}{\Omega}}$$