There are three variations on this phrase that I can find in Google, and I'm
not sure whether all three are really used, or how frequently. But I am
confused about the differences between them.

> 大{たい}したこと **では** ない

I believe this means "(It's) not a big deal." Ok, I understand.

> 大{たい}したこと **は** ない

Isn't this different from the first sentence? It seems to mean "There aren't
any particularly big problems."

> 大{たい}したことない

This one could go either way.
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2019-02-05T21:05:45.147
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2022-01-27T18:00:45.063
2020-02-01T19:16:02.980
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question
particles expressions
Confusion about 大したこと(で)(は)ない
Your understanding of the first two variations is correct. The sound quite
formal, maybe something like:

  1. "This is nothing important, really" 
  2. "There is nothing important, really" 

Last variation is more colloquial, maybe something like:

  3. "No big deal"
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2019-02-06T04:03:01.180
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2019-02-06T08:00:33.230
2019-02-06T08:00:33.230
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answer