Why there are tie lines for 3 phase equilibrium however not for 2-phase equilibrium in a two-component system?

Lever rule derived from mass balance gives the relative amounts of the co-existing phases. It is applied as follows - the tie line is treated as a lever arm, with the fulcrum at the overall composition.  For  the  arm  to  be  horizontal,  the  weight  to  be  hung  at  each  end  must  be proportional  to  the  arm  length  on  the  other  side  of fulcrum.  Weight at each end corresponds to the amount of the phase at that end. At temperature T and overall composition co,  the  relative  amounts  of  liquid  and  the  solid  phases  are  determined  as  follows  - Expressing the weight fractions of liquid and solid as f_i and f_o