We also need to know the firmware version you are using to run your laser. We have seen the Eleksmaker machine ship with an older version of grbl so if this is the case, go with the GRBL-M3 device in LightBurn. Ideally, you would upgrade to GRBL 1.1f and then use just the GRBL device.
Check your setting for $30 and $32 in the console by typing $$ and pressing enter.
$32 should be 1 (enable laser mode)
$30 should be 1000
The $30 value can be set either in your firmware to match LightBurn or in the LightBurn device settings (S Max) to match your GRBL setting.
The working area of your system is defined in both LightBurn and your firmware. Here are the latest docs for configuring the firmware. https://github.com/gnea/grbl/wiki/Grbl-v1.1-Configuration
Here is another great resource for your setup.
http://wiki.eleksmaker.com/doku.php?id=elekslaser_a3_pro
As another member previously posted, "Scroll down to the bottom "operating instructions" And follow that, once you have it working in elekscam. It should work in lightburn. Follow the instructions to the letter and that will get you up and running…"
There is a separate group for these types of machines on Facebook and quite a few people there use LightBurn, so they would be able to advise you more. Facebook Support Group for Eleksmaker - https://www.facebook.com/groups/107222596696733/