I'ma do it
"I'm going to do something" is often contracted in the United States. The from "I'm gonna do something" is universal, but there are a couple more contractions:
I'm gon' do something
I'm 'na do something
I'ma do something
I would like to know which of those contractions are commonly used in the UK as well.
Maybe the first if you're really lazy. Not the other two.
the first sounds american southern
never hear the second
third seems common everywhere
"I'm finny to do something/I'm a fi'n' to do something/I'm a fin'a do something" is a different, though closely related construction. It is of course a forbrokence of "I am fixing to do..."
(and 'forbrokence' is of course a corruption of "forbrokenness")
Your 2nd and 3rd variations are common. In fact, they are described in a book whose name I forgot.
I'm an American and a native English speaker and I've never heard anyone talk like that.
It's mainly a Southern, African American slang.
Until I moved to NC, I had never heard of it either. Very Kuntry.
I've never heard any of those variants in Britsh speakers, but I'm familiar with them as used by (some) American speakers, from films and the like.
I've only ever used #1 to make fun of hicks. "I'm 'na do something" doesn't come out like that, does it? It's more like "I'muhna do something." #3, I at least know of, I don't really use it except to joke around, though.
US, Arizona.
Another variation of Leasnam's examples:
"I finna do something." (Or, if the speaker is even more lazy, "I f'n do sumpin.")
Everyone I know uses #2 constantly.
California
I hear the second one a lot in Wisconsin.
I've certainly never heard any of these examples used in England.
Some people use 'gonna' from time to time, but it's generally regarded as sloppy. I'd strongly advise anybody wanting to be respected to use the good old-fashioned 'going to'.
Around my hometown in East Texas people use No. 1 and No. 3 all the time. Here in New York, people say "I'm gunna" or something close to No. 2 which actually sounds like "Ahməna". (with a schwa for the second syllable).