In the equation  ${\text{Ca(HCO}}_{\text{3}}{\text{)}}_{\text{2}}+{\text{Ca(OH)}}_{\text{2}}\to 2{\text{CaCO}}_{\text{3}}+2{\text{H}}_{\text{2}}\text{O}$the F-number of element 3 are            

Calcium bicarbonate solution reacts with calcium hydroxide solution to form Calcium carbonate and Water.

${\text{Ca(HCO}}_{\text{3}}{\text{)}}_{\text{2}}+{\text{Ca(OH)}}_{\text{2}}\to 2{\text{CaCO}}_{\text{3}}+2{\text{H}}_{\text{2}}\text{O}$            

F-number: “The frequency of occurrence of various elements in an equation is called F-Number”