I tried installing Linux Mint 18.3 on a ThinkPad T420i yesterday, and ran into a serious kernel problem that forced me to revert to Mint 18.1. The problem was a hang that is due to a bug in the driver for Intel graphics chips (such as that used in the T420i). The hang caused the desktop to freeze at random times, sometimes permanently, requiring reboots using Alt-SysRq magic keys.
Occasionally the hang appeared after logging out, and several messages like the following would appear on the text console:
[Wed Mar 21 12:02:00 2018] [drm:drm_atomic_helper_commit_cleanup_done [drm_kms_helper]] *ERROR* [CRTC:32:pipe A] flip_done timed out
There are a number of web pages describing the problem, such as this one. The suggestions on some of these web pages, involving adding kernel boot parameters, did not work for me. As mentioned above, I chose to revert back to an installation of Mint 18.1, which uses an earlier kernel (4.4) that does not have the bug.