There's no mechanical/grammar reason but the first one is used sarcastically. When someone thinks something is a big deal/great/awesome and you say back "well that's certainly a thing" it's implying that the only thing you agree on is that that thing exists, not that it's great.
The other two are nonsense and some people just find that funny.
"That's certainly a thing" refers to phrases such as "You definitely said words." It expresses that one wants to acknowledge that something happened which demands commentary, but the commentary is self-evident, and thus sarcastically skipped.
I'm not a native speaker and have a few questions about the first point:
There's no mechanical/grammar reason but the first one is used sarcastically. When someone thinks something is a big deal/great/awesome and you say back "well that's certainly a thing" it's implying that the only thing you agree on is that that thing exists, not that it's great.
The other two are nonsense and some people just find that funny.
Thank you for the explanation!
"That's certainly a thing" refers to phrases such as "You definitely said words." It expresses that one wants to acknowledge that something happened which demands commentary, but the commentary is self-evident, and thus sarcastically skipped.