^A horizontal pull on three blocks

^A horizontal pull on three blocks

Using NSL, we can write

System m₁: m₁ a = F – T₁₂

System m₂: m₂ a = T₁₂ – T₂₃

System m₃: m₃ a = T₂₃

System total mass: (m₁ + m₂ + m₃) a = F