Xperia Z3 boot loop after TWRP installation
Now that you can enter into fastboot, you can execute the commands below
fastboot flash recovery ~/Downloads/bootZ3monxtest01.img
and fastboot reboot and install a custom rom.

flash boot boot.img is used to flash kernel images.
boot.img is a file which contains kernel and ramdisk.

More on Boot &amp; Recovery images can be found here