All of these pokemon are similar in the regard that they were all introduced in Generation 3 and have BSTs of 380 each.
I love Mawile. It's so adorable, and has a pretty damn good movepool. It's only hinderance is its low stats, although it's attack and defense stats are quite usable, especially in the NU metagame..
Sableye has the useful ability of not having ANY weaknesses in the entire game, a trait shared with Spiritomb. Well, that is, no weaknesses if the opponent doesn't have Foresight...
Delcatty is the more... impractical of the three, in my humble opinion, but has a pretty good movepool. In my opinion, however, it doesn't make up for its abysmal stats. Unlike its distant cousins, it needs a stone to evolve. A one of a kind stone, in that only one can be found in the game it can be acquired in.