Removing the CMOS battery will reset the system clock, but it will not erase or reset the BIOS password.
HP has a backdoor password that can be used to reset the BIOS password on certain models, including the EliteBook 840 G3. Here's how: How To Reset Bios Password Hp Elitebook 840 G3
This public link is valid for 7 days and shares a thread, including any personal information you added. This link or copies made by others cannot be deleted. If you share with third parties, their policies apply. Can’t copy the link right now. Try again later. Removing the CMOS battery will reset the system
If software methods fail, hardware-level intervention is required. These carry high risks: Reset HP EliteBook 840 G1 BIOS Password - FAQ - JustAnswer This link or copies made by others cannot be deleted
Look for specialized BIOS unlocking services that support HP EliteBook 840 G3.
Open the laptop base and locate the SPI Flash ROM chips (usually an 8-pin Winbond or Macronix chip near the CPU or chipset). The 840 G3 typically utilizes a 16MB (128M-bit) main BIOS chip.