Accessing an MCU via minicom but it keeps saying Resource Busy I am accessing a micontroller that's running linux off the SD card and is connected to my Mac via USB cable, but when I try to access it serially via minicom, it errors out:
minicom: cannot open /dev/disk3: Resource busy

How do you know what application is using this disk? I tried using lsof but not sure if I find something that's using disk3. What am I missing?
It seems like you have confused some things:
minicom is a program that will allow you to communicate with for example a different computer running Linux over a serial connection.
/dev/disk3 is a device node on your computer that refers to a physical block device (i.e. for example a hard drive or an SSD).
It does not make sense to ask minicom to communicate with an SSD disk. This is why your command fails.
Instead when using minicom you need to use the device node for a serial communications port. For example it could /dev/cu.usbserial or similar depending on the type of serial port you have.