You can extend the 180 days x 6 times. (180 days * 6 = 3 years).

Open PowerShell and run this command, which will give you your evaluation/license/trial data:

slmgr -dlv

When the trial comes to an end, run slmgr -rearm to extend it by another 180 days.

slmgr -rearm

Next restart your computer via the GUI, or this Powershell command.

Restart-Computer

Once restarted, open PowerShell and check your trial settings.

slmgr -dli