Unfortunately converting Genesis 8 clothing packages to Genesis 3 isn't easily done. You have a few options in DAZ however.
Sometimes using Genesis 8 clothes on Genesis 3 just works, you just have to disable the contextual filter in the Smart Content pane.
Using an asset like
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or
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, this works most of the time. The last one is only for females only, unfortunately.
You could also change the compatibility base of a Genesis 3 figure to Genesis 8. Just select your Genesis 3 figure, and choose Edit > Scene Identification from the display options (play arrow overlaid on a hamburger menu icon) in the Screen pane, you can set the Compatibility Base of a figure to anything you want there. The could lead to some weirdness or it might just not work correctly, but it's worth a try.