val='a,b,guido'
val.split(',')

['a', 'b', 'guido']
pieces=[ x.strip() for x in val.split(',')]
pieces

['a', 'b', 'guido']
'::'.join(pieces)
'a::b::guido'

first,second,third=pieces
first+'::'+'::'+second+'::'+third
'a::::b::guido'