解：$(3)$有两种情况，如图
$(a) P_{ }=\frac {U_{ }^2}{R_{ }}=\frac {({ 6 }\ \text {V})^2}{{ 10}Ω}={ 3.6}\ \text {W}$
$(b) P_{ }=\frac {U_{ }^2}{R_{ 1 }}+\frac {U_{ }^2}{R_{ 2 }}=\frac {({ 6 }\ \text {V})^2}{{10 }Ω}+\frac {({ 6 }\ \text {V})^2}{{15 }Ω}={ 6 }\ \text {W}$
