100/50/50 - you have 10 soldiers in a town with 5 foot walls/moats. Infantrymen can easily climb over and catapults can lob rocks over.
50/100/100 - you have 5 soldiers in a town with 10 foot walls/moats. Infantrymen have a harder time climbing over, and catapults need to be in a better position, but you have fewer soldiers on the inside.
in actual words:
The Defence and Special Defense stats modify the amount of damage taken; the HP is just the pool that that damage is logged to.
Using the analogy above, it becomes obvious that 50/100/100 is better than 100/50/50. That's why Toxapex is good (one of the reasons) and why Wailord is bad (one of the reasons.)
