Only the character's highest Circle is used to determine their Death Rating.
For the purposes of determining which missions are within range, a Multi-Discipline character’s Circle Rating is equal to their highest circle, plus half their second highest circle, plus one third their remaining circles (always round down any remainders). For example, a character who is Illusionist 3/Elementalist 2/Troubadour 1 is considered 4th Circle (3 + 2/2 + 1/3) and may therefore volunteer for 3rd, 4th, or 5th Circle missions.