It is how much it weighs (grams per square meter). There is a correlation between weight and thickness, but it won't be 100%, as some papers are more dense than others. Having not used two of those three, I can't suggest how the thickness compares.
Are you sure about the info on Kentmere RC? Where did you find this info? It seems incredibly heavy for RC paper. This is approaching museum weight in FB.
I have just taken another look at link to the Kentmere site on the Harman site. It looks as if Kentmere mention the paper weight of 190 then the polyethylene of 40g per side giving the 270 g/sq metre.
Harman papers seem to mention paper weight of the same 190 only and does not include the polyethylene weight.
So I admit I am a little confused but I am assuming that Harman with its polyethylene base will have the same overall weight as Kentmere.
It's tricky when two manufacturers( now one of course) describe paper weights differently and we may be comparing apples and pears.
Anyone who can nail this down unequivocally is invited to respond. Kentmere even mention thickness which is listed as 245 followed by the Greek letter "mu" then m. I can't find any such reference for Harman paper thickness.
Just to muddy the waters further, it says VC Select is coated on a thicker base but thicker than what?