How do I lock my bootloader again? I tried to root my Moto G3; but I was unsuccessful. Now my phones boots with this warning. 

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I am sick of seeing this on boot screen. How do I disable it? Seems like I need to lock the bootloader again. How do I do that? I'm completely blank.
First, I want to tell you that relocking the bootloader will NOT get rid of that message... On the Moto G3, once the bootloader has been unlocked that message will always appear, forever. That being said, you can flash a custom boot logo to cover the warning. I will explain how to relock the bootloader and fix the logo though.

To successfully relock the bootloader on a Moto device, you must flash a complete stock firmware image via fastboot that is of the same or newer revision than you have installed, you cannot downgrade to relock. Firmware images are available here and vary by exact model. Once you have that unzipped and in your fastboot directory, perform the following commands:

fastboot oem lock begin  
fastboot flash partition gpt.bin  
fastboot flash bootloader bootloader.img  
fastboot flash logo logo.bin  
fastboot flash boot boot.img  
fastboot flash recovery recovery.img  
fastboot flash system system.img_sparsechunk.0  
fastboot flash system system.img_sparsechunk.1  
fastboot flash system system.img_sparsechunk.2  
fastboot flash system system.img_sparsechunk.3  
fastboot flash system system.img_sparsechunk.4  
fastboot flash system system.img_sparsechunk.5  
fastboot flash system system.img_sparsechunk.6  
fastboot flash system system.img_sparsechunk.7  
fastboot flash system system.img_sparsechunk.8  
fastboot flash modem NON-HLOS.bin  
fastboot erase modemst1   
fastboot erase modemst2   
fastboot flash fsg fsg.mbn  
fastboot erase cache   
fastboot erase userdata  
fastboot erase customize  
fastboot erase clogo  
fastboot oem lock    
fastboot reboot  


Note that the number of sparcechunks can vary by model and Android version from 5-11 pieces, but what you flash must match the firmware image you have. If any piece fails, the bootloader will not lock.

Once this is done, you will have a completely stock device and a relocked bootloader, with a nice Bootloader Unlocked warning screen. As I said, once the bootloader has been unlocked, it cannot be restored to it's original condition in any way we know of.

To get rid of the screen (but not the delay or vibration, we can't get rid of that) you can flash a patched logo.bin file in fastboot. This can be done even with a locked bootloader. This can be done by flashing the logo.bin file from this ZIP file with the command:

fastboot flash logo logo.bin

And the warning screen will disappear. The ZIP file can also be flashed in it's entirety via TWRP recovery instead. 

All of this may not be relevant, since rooting this device is very simple, since that wasn't your question I will not answer it here, but I will point you to my thread on XDA which may be of assistance if you want to attempt rooting again. Note that the logo.bin file I reference above can be flashed on a stock or modified system to get rid of the warning, your bootloader does not have to be locked to get rid of the warning.