Bert,
you can't compare these three classes this way. These are apples, oranges and pears.... They are all fruit but that is about it.

The F18's specifically choose that build weight because it was doable, low tech and they did not want a development push on weight and materials. Sailors are right to demand a boat that measures under the class weight and needs correcting to be legal....(that is part of the class's apeal)

The F16 class rule is similar but the founders wanted more development and lower production weights (to gain some market share from the F18 class)... the three builders you mention all take a different route to their market solution ... This is a function of their F16 design rule and core philosophy. The racers in the class don't care and declare all solutions equal and one up and two up flavors equal as well... (whatever!!) You can't demand an F16 measure into a non existent standard as a consumer.

(The measurement rating systems just measure each boat and issue a unique rating for specific build eg Viper ... The default F16 rating would be for the lightest boat allowed by their rule...)

The N17 class is SMOD and ISAF will go NUTS if Nacra changes the platform build over it's Olympic cycle... They had enough of the Tornado sailors working around the Tornado One design rule to game the system. You can't even drill a hole in the N17 platform.. If the N17s differ in 2 years... There will be hell to pay.. (Now, new boats could well be stiffer then used boats and that should drive the elite to stay with new boats)

If you are a 20 to 40 year old sailor looking at one of these three classes.. You have clear choices. Which one(s) survives for 20 more years like the Thistle or Lightning or Hobie 16.. ... Hmmm. YMMV