Reactivity in periodic table

Amoung three elements F,CL,Br which is more reactive? I guessed it to be bromine because it has three shells and hence when we go down the group on the basis of their shells bromine will easily lose electrons.
But the book says it is fluorine since it can easily gain electrons.And how can that be? Shouldn't we classify something as reactive on the basis of electropositivity?
Looks odd because another answer to a question similar to this answered on the basis of electronegativity.
What exactly is the criteria I should use here?