Under what circumstances would you prefer one of the below over others?
a) Get hold of, b) Get ahold of, c) Get a hold of
The three variations of this expression exist and are acceptable.
The meaning actually depends on what follows of, so get hold/ahold of someone means communicate with/reach someone and get hold/ahold of something means obtaining/literally reaching out for something. And I believe they convey the same meaning, with "get ahold of" being spoken English (apparently because it's easier to pronounce) for "get hold of" and "get a hold of" being specifically used for physical actions.
"Get ahold of" doesn't exist. "Get hold of" and "Get a hold of" are mostly interchangeable, but "get hold of" is more often used with people "get hold of Mr. Jones and tell him..." and "get a hold of" is used with things, like gaining expertise "I think I've finally gotten a hold of this subject..."
EDIT OK, new information: ahold does exist, but it's a slang.Thanks, @NewAlexandria
"get a hold of" is the only proper form. I would prefer this in all circumstances since it is correct use of the language.
"Ahold" is not a proper word.
"ahold of" is a colloquialism, but the OED does not define "ahold" as a word out of context of the colloquialism.
"get hold of" is just a broken form, where the speaker couldn't manage to use the article "a".
I suppose I would use the incorrect forms in social contexts where being seen as a 'non-native speaker' would put me danger (perhaps if I was confronted by gang members)
I think that grammatically speaking, get a hold of yourself doesn't make much sense - and in fact it seems to me that people originally recognised this.
That linked NGram chart shows how the more "logical" get a hold on yourself was actually more common when both expressions started becoming widespread in the 20s. And this one shows that plain get hold of yourself has always been more common than either version with the word "a".
Whilst I don't disagree with other answers saying that "get ahold of yourself" is "incorrect", that link to over 15000 written instances shows that in the minds of many, it really is a kind of "adverb" that gets round the inherent grammatical problem in what's now an established idiomatic usage. Somewhat akin to, for example "Get along with you!" (meaning "Go away!", "Be off with you!").
EDIT: Per comments below, I would just note this one semantic distinction...
There's a certain amount of flexibility in the above constructions (dialectally, at least, #1 could use ahold, and it could carry the sense of #2). But the #1 make contact sense always needs of, and if you try replacing hold with grip, grab - with or without preceding get [a] - it should be clear that idiomatically, certain forms either don't work at all, or have restricted (probable) meanings.