We have to use the same element for all three rounds, yes?
Does the mark card have to be the chosen element?
Not for all rounds. From the rules: "That will be your Main Element during
that round."
To clarify, you must use the same Main Element against each opponent, but you can switch your Main Element between opponents.
For example, if you start using Time as your Main Element against kaempfer13 in Round 1 game 1, then you must use Time in game 2 and game 3 too against him (still in Round 1). In your next duel, you can change your Main Element if you want to.
The deck mark can be anything.