STERN : 1. very serious especially in an unfriendly way
a stern judge ; a stern warning
2. expressing strong disapproval or criticism
He gave me a stern look.
3. not likely to change or become weaker
stern determination/resolve
STRINGENT : very strict or severe :
stringent training ; stringent budgetary constraints — used to describe a command, rule, etc., that must be obeyed ; stringent rules/requirements/restrictions/standards
STRICT :1 — used to describe a command, rule, etc., that must be obeyed :strict rules/laws/regulations : They want to impose strict limits on government spending. ;We are under strict orders not to leave the prisoner alone.
2 . demanding that people obey rules or behave in a certain way
a strict teacher ; Her parents aren't very strict.
3 . : carefully obeying the rules or principles of a religion or a particular way of life
a strict Hindu ; a strict vegetarian
4. : complete or thorough
This project requires strict (=absolute)secrecy : He insists on strict adherence to the rules.
5. : completely correct : exact or precise
He's not a volunteer in the strict/strictest sense (of the word) [=he is not really a volunteer] because he receives a small stipend.