I left a recent 3 month part time online job off applications

Hi everyone, so I've been applying to jobs the past few months and honestly didn't even consider my brief stint with Lionbridge when I filled out applications. I was an Search engine evaluator and before my performance improved, they canned me. It was back last summer and it was online and part time - 20hrs a week at most. So is this bad? I should also mention that, operating off my full time work history, I selected no when asked if ever terminated. My question is, do I bring this up now in an interview or see if it's brought up in a background check ? I've read where some say it's not a big deal and others where people are fired months later when it comes up. Please help, I have a interview next week and am freaking out.

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