Arrange F, Cl, Br in order of decreasing electron gain enthalpy (most negative to least negative).
Electron gain enthalpy becomes less negative down group 17 due to increasing atomic size. Cl has the most negative value due to optimal nuclear charge and size, followed by F (less negative due to electron repulsion in its compact 2p orbitals), then Br. Order: Cl > F > Br.
Ref: NCERT Class 11 Chemistry > Chapter 3: Classification of Elements and Periodicity in Properties > Topic: Electron Gain Enthalpy and Electronegativity and Valency