Decrypt the following messages encrypted using the Caesar cipher: $f(p)-(+3)mod20$

Alphabet: A$,$B$,$C$,$D$,$E$,$F$,$G$,$H$,$I,J$,$K$,$L$,$M$,$N$,$O$,$P$,$Q$,$R$,$S$,$T$,$U$,$V$,$W$,$X$,$Y$,$Z

$($a$)$ EXVB KRVSLWDO

$($b$)$ HDW GLP VXP

$($c$)$ XIAFONGD