I personally feel that those three characters just bloat out the plot and with out them the story would have better pacing. That's not to say I want to see them dead, but I wouldn't mind if they were gone... it's not like their core jokes have really evolved all that much, and after so long they just don't do anything for me.
Personally I would like to see chopper replaced with someone more like stein from soul eater. A doctor that is actually brilliant, but just has a few things off with him and at times can be very evil. Choppers role of a doctor has importance, but the character itself adds pretty much nothing but merchandise sales at this point.
As for Brook, I don't hate him, but he doesn't really add anything but filler in general.
Sanji... if you removed his female gag stuff I would actually really enjoy his character (but it's a main theme in non serious stuff, and in the serious stuff he most of the time gets pitted up against a female and has some of the most pathetic fights in the series.... about the only sanji fight I can recall enjoying to any degree was his fight with bon clay), but that stuff is so grating at this point that any time something like that comes up I hope his nose bleed is strong enough to write him off the show anymore :/