Three blocks of masses $m_1=1\mathrm{kg},m_2=2\mathrm{kg}$ and ${\mathrm{m}}_3=3\mathrm{kg}$are connected by massless string and placed on horizontal frictionless surface as shown in fig. Force F = 12 N is applied to mass m₁ as shown. The contact force acting on mass m₃ is