Basically, we have to follow only https://elinux.org/RPi_U-Boot

Make u-boot on Ubuntu 18.04

Create u-boot.bin from source code.

git clone git://git.denx.de/u-boot.git
cd u-boot
sudo apt install gcc-arm-linux-gnueabi
sudo apt install gcc-aarch64-linux-gnu
export CROSS_COMPILE=aarch64-linux-gnu-
make rpi_3_defconfig
make -j1 -s
sudo cp u-boot.bin /media/user/boot/kernel8.img

Copy u-boot image to SD micro card.
Copy u-boot image to boot partition as kernel8.img.
SD card is already installed raspbian.

sudo cp u-boot.bin /media/user/boot/kernel8.img

Connect Rapsberry Pi 3 and PC

Raspberry Pi 3 B pin assign is as following
* PIN 6 GND
* PIN 8 TX
* PIN10 TX

We should confirm boot raspbian and enable GPIO.

Serial Setting on PC
* Boolate: 115200
* data: 8bit
* parity: None
* Stop: 1bit
* Flow control: Nonee

After Raspberry Pi boot, We can find u-boot pronto on serial port.