It will work only if you use LastPass to store the partial password, and then you will have to manually add the 3 characters during login. LastPass won't be able to successfully do auto-logins using this method -- but you already know that.
However, why give a malc0der access to most of the password ? It's insane - with only 3 characters needed to complete the complete password they will crack the entire password using softs designed to do so within a single day - probably only hours or even a few minutes.