Please check this post. Or any other sdmmc3 post on nano forum. This is a very common question here. Not new issue.
If you have any problem after modifying dts, share the dmesg, full dts (de-compiled from dtb).
As my previous comment said, the only thing you need to care about is vmmc-supply, cd-gpios and if your vmmc-supply is a always-on, you need to add the always on property in your sdhci.