Even if only 2 artists come out in a round, you still put out the third marker. It goes to Lite Metal (yellow) or whatever the lowest numbered artist that didn't have cards showing.
Did the rules for "Modern Art: the Card Game" (with Lite Metal - it's Vermeer in "Masters Gallery") state this explicitly?
One might infer your statement from the "Masters Gallery" rules. They say that Value 1 Token is assigned to the Artist Card with the "third-most displayed Masterpieces" and that ties are broken in favor of the artist with the lowest number.
Personally, I would agree with you: if only two artists have a non-zero number of Masterpieces displayed, then the other three artists are all tied with zero - and the tie would be broken as you've described.
But I know that others have argued (in discussions of other games) that zero is not enough for most. In Rococo, "the players with the most and second most Dresses in that hall receive Prestige Points"; this is from the printed rules (there is a tie-breaker). This question of "can zero be most?" came up, and the designer ruled that it could not.
It's better when the printed rules address this question explicitly.