C-H Bond Dissociation Energy varies with substitution
Nature always try to acheive least potential energy for stability . Any organic reaction which makes the product less stable is not favourable . 

If the C-H bonds are cleaved homolytically in all cases , the resulting stability of the free radicals will tell us about the feasibility of the reaction . 

The primary free radical is the most unstable and to bring about its C-H bond breaking would mean increasing its energy comparitively . Hence its bond energy is the most and so its the stronger bond of the three .  

Tertiary free radical is the most stable while secondary lies in between them . The same explanation can be used to draw out their comparitive strength of the C-H bonds .