According to your own data you provide, the MiG-21 has the longer range of the 3...
As to the J7 having shorter range, I'd not be surprised if the engine the Chinese use is less efficient than the Soviet one it was copied from (and it is indeed an almost carbon copy of an early model MiG-21), and the same'd be true for many other of the aircraft's systems (my guess would be for example that the MiG would have thinner metal and flush rivets, rather than thicker metal and round headed rivets on the more primitively built J7, making it lighter and more aerodynamically efficient).
The JF-17 is a more modern development of the J7 concept with improved materials, engines, aerodynamics, giving overall better performance.
Mirage III and MiG-21 were both designed for a very similar mission: short range rapid response interception of incoming enemy strikes (and especially bomber strikes) with air to air missiles only (guns being added pretty much as an afterthought, neither aircraft was designed with the agility needed for a dogfighter). They didn't need long range, the idea was to get up to altitude as quickly as possible, launch missiles at the approaching enemies, then get down again to refuel and rearm and do it all over again. Later variants were given increased range and agility to allow them more versatility, but that was the original concept behind both aircraft.