Higher the electronegativity, higher is the acidity..then why is acidity of BBr3>BCl3>BF3??

the same is happening due to the phenomenon called as BACK BONDING , due to back bonding the electron donation by flourine to boron , electron density increases in boron , and hence its lewis acid charecter decreases . 
inorder to acheive octect around boron , which gives stability to the molecule . 
as we move F TO CL the size of the orbitals will increases , and hence there is not much effective back bonding in case of bcl3 . 
HOPE IT CLEARS 
ALL THE BEST ..