This is exactly the same issue that I had, except in my case the last 5 digits were flashing as opposed to just three in your case. I contacted Elksmaker by email at email@example.com and they were able to diagnose the fault to being an issue with the LED board under the first flashing set of digits.
To confirm the issue you just need to remove the bottom plate and bypass the first flashing board to see if the rest of the LED's past the initial flashing board return to normal.
They sent me a replacement board free of charge. I replaced the board and all the digits returned to their normal mode.
I could not fault them with this.