Hi FrankPCP,
Sorry that I think we didn’t know how to help here since there is no experience for us to connect emmc on sdmmc3.
If you refer to tx1 TRM ch32 SD/MMC controller, there is a section “Pinmux Options”, as you can see sdmmc4 is using a config named “emmccfg4”. However, sdmmc3 is using different config which cannot be changed.