PadZero in string Parameters Houdini

create string parameters keyframe
then edit expression

Python:
hou.node("..").name() + '_%03d' % hou.intFrame()

hou.node("..").name() : parent name 

*/ works for Integers  Floats and Strings*/ 
 '_%03d' % hou.intFrame() : pad3 decimals places  using Frame as base number    

****another method only for strings*****:

n = '4'
print n.zfill(3)
>>> '004'

HScript:

opname("..")+"_" + padzero(3,"$F") 

opname(".."): parent name 
padzero(3,"$F")  :pad3 decimals places  using Frame as base number