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I recently overheard someone estimating a number as

three point ish million

when discussing the maximum damages in a law suit.

Do native speakers use "ish" in this way? (My gut tells me no.)

The standard pattern for 'ish' is that is changes a adjective to an adverb (like 'like' or '-ly') which addds the meaning 'around or 'roughly' or 'nearly'. For example,

'bluish' for 'with a blue tinge.

It doesn't work with all adjectives, eg 'terrible-ish' just doesn't work for 'terrible or nearly so'. (though possibly informally).

The current popular informal extension to this pattern is that the suffix 'ish' has an additional use as a stand alone word, a sentence adverb.

The instance you overheard

three point ish million

sounds like it is trying to slightly move over to mean 'a vague thing that I'm not specifying'. In current culture it seems to be the trend to do all sorts of weird, conversions of noun to interjection to past participle subjunctive verb ending. Some of them work, some of them not. For me, this one doesn't but maybe it'll catch on.

It took me a couple of readings to 'get' it. 'ish' is not a formal thing so the patterns it appears in are pretty unstable. I can see how with some semantic drift, 'ish' works here, but this instance still sounds currently 'off'. As a native speaker, I would suggest that non-native speakers not attempt to use 'ish'.

While "three point ish" makes sense, I'm not sure what it's supposed to mean before "million." That's because the phrase "three point million" doesn't make sense. Maybe what you actually heard the native speaker say was something like "three point two ish million" or "three million ish" or "three million points ish."

Anyway, what it appears you're driving at is the use of "ish," and, yes, that's become acceptable in informal English.

In recent years, "ish" has started to appear as a word in and of itself as people say it after what's been said, either by them or someone else, to mean that it may not be exactly right or true but is close enough for the purposes of the conversation, is their best and most educated guess of what's true, is somewhat true, or the truth is somewhere thereabouts if not exactly so.

Example 1:

"Are you hungry?" asked Henry.

"Ish," answered Susan.

Example 2:

Oh, yeah, you'll like him. He's handsome. Well, ish.

Additional sources:

https://www.urbandictionary.com/define.php?term=ish

https://slate.com/human-interest/2014/06/ish-how-a-suffix-became-an-independent-word-even-though-it-s-not-in-all-the-dictionaries-yet.html

Addendum:

In researching your question and after reading of another use of "ish" I didn't know about, your question really needs more context, even if just what accent the person spoke in.

While I am American and haven't heard this myself, there is apparently an informal usage of "ish" (see def. 3) in American English as a euphemism for the expletive "s-h-i-t."

With that in mind, I now find it plausible that "ish" in that exact context could've been used. I could, for example, easily see someone saying something like "It cost three point ish million" to mean "It cost three point s-h-i-t million," or in other words "some s-h-i-t sum" (i.e., "inconsequential amount" or "petty amount") over three million dollars.

Therefore, "three point ish million" could be conveying three million, along with some beyond-the-decimal-point amount over exactly three million that the speaker is vulgarly, if euphemistically, dismissing as not worth remembering or specifying.