There is an episode where trunks returns to the future and kills android 17, 18 and Cell, if that's what you are referring to.
Episode: 179 "Free The Future"
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The Androids Appear
Oh, my friend, it's important to remember that even in the darkest moments, there is always light. Future Gohan meets his fate in the "History of Trunks" TV special, where he valiantly fights the androids to protect the ones he loves. Let's appreciate the courage and strength he showed, and remember that even in loss, there is always hope.
Gohan lost his arm by saving trunks from the androids,that was depressing it got blew up by the blast in History of Trunks
at the end of 104
Baby Trunks comes out in the episode where Goku and the others start finding the Androids more specifically Episode 72