Where do I get my saved messages on my Samsung j3 after a factory reset

1. Even assuming that they were still there, they're in a database file, so you'd need a SQLite editor and you'd have to understand SQL well enough to find them.

2. A factory reset wipes them out, so they're no longer there. That's the whole idea of a factory reset - nothing is recoverable after you do that. (Imagine if you did a FR before selling your phone and the buyer could "resurrect" your deleted messages - and some of them were "not for public use". Or the banking password that you saved wasn't erased.