Basic attack used (removed skill from skill slot). Non-elemental.
Basic, non-elemental attack without legacy Triumvirate: Elemental Exposure does not trigger. The visible effect above the enemy does not occur.
Basic, non-elemental attack with legacy Triumvirate: EE triggers, the visible effect occurs.
Elemental attack (1 Element) without Triumvirate equiped: EE triggers, the effect occurs.
Legacy Triumvirate does affect EE, though I cannot reliably tell you if it works for more than 1 element, as one element already triggers EE (as confirmed in Test 3). It might also be that Legacy Triumvirate does 1 Elemental damage (from the base damage attribute) and this triggers. We cannot be sure here. However: Damage-wise the lowest I've seen with EE deactivated (switched the passive) was ~19k damage, the lowest I've seen with EE activated was ~22k damage. This fits nicely with 15% damage increase from EE (3 elements).
a) Anything I could've forgotten in my testing setup? Anywhere I could've gotten elemental damage from?
b) If not: I would like to have this confirmed by other testers. Remove your gear and your basic attack skill and test with EE and the Legacy Triumvirate and confirm the numbers I've seen.
c) If this turns out true, legacy Triumvirate should be, hands down, the best source. 15% multiplicative damage for any attack you do with any skill you do without having to worry about anything should pretty much beat Loot 2.0 Sources all the way. Is that the case?