ahold is wrong
a hold is correct, as is hold

Well, do they mean the same?

ahold is informal, maybe regional or a bit outdated.

One dictionary:
a·hold
[uh-hohld] Show IPA
noun
1. Informal. a hold or grasp (often followed by of ): He took ahold of my arm. Grab ahold!

Another:
get/grab/take ahold of something
to take something firmly in your hand and hold it tightly there.
Grab ahold of the rope!

I don't like, grab a hold of sth.

Thx. But do "grab hold of" and "grab a hold/ahold of" mean the same in any context?

Yes, usually or always, I would say.
What's your example?

to me, "ahold" is colloquial and "a hold" is formal. All three of your versions in the OP, however, mean the same thing, IMO. Besides, the difference between "a hold" and "ahold" is only recognizable in writing, not in speech (usually, anyway). :-)

I think it's usually written 'ahold' as one word, as the dictionary excerpt shows.

However you write it, there's no difference in meaning, only a difference in register.

standard:
grab sth.

colloquial:
grab hold of sth.

regional/dialect:
grab ahold of sth.

In some regions it might even be pronounced 'aholt,' so you might see that phonetic spelling as well.

'Hold' can also be a noun, meaning grip, purchase, which is where 'a hold' would be possible.

The mountain climbers couldn't gain a hold on the smooth rock surface.

(Not a very good example, sorry.)

Many thanks!

FWIW, my enormous Webster's Third New International Dictionary (printed version, 2002) says this:

ahold, also aholt (probably from a + hold n.) dial.: a hold (take ~ of) (get ~ of)

Okay, thanks. That's what I call 'Webster's 3rd unabridged,' but mine was printed several decades ago. (-:

I just found it interesting that it actually mentions "aholt." :-)

Yep. (-:

I would place that as maybe Appalachia? If I had a guess coming, as people say in my dialect. But that's a total guess.

D.A.R.E. (the Dictionary of American Regional English) would probably be the place to check -- Norbert used to have a copy, but I don't know if it's available online.