These are all different ships...from a distance, no, but up close, definitely.
Essex - One catapult, early bridge, single 40mm Mk 2 on fantail, no 40mm galleries on starboard side, portside hangar deck catapult.
Yorktown II - Two catapults, extended bridge, twin 40mm on fantail, no 40mm galleries, no hangar deck catapult, instead replaced with 40mm guns. Yorktown kit also corrected some errors in the original Essex, such as the turndowns on the flight deck.
Franklin - Two catapults, no hangar deck catapult, extended bridge, relocated radar platform (and possibly a new array, from SK to SK-2), twin 40mm on stern, and 40mm galleries along the starboard side.
Of course, your aircraft are going to change as well...
Essex - F6F, TBF, SBD
Yorktown II - F6F, TBF, SB2C
Franklin - F6F, TBF, SB2C, F4U-1(after Jan. 45)
All three ships are the same class, true, but each represents different time frames of their careers, early war, mid war, and late war.
I should also mention that Essex and Yorktown both carried 5 radio masts during their careers, and Franklin carried 4 until January of '45, after which time she carried only 2.
Regards,
Jeff Herne
Modelwarships.com