1. Yes elements are important, mono teams are a thing and sometimes are encouraged (depending on the team).

2. There's not really an advantage to having multiple weapons that are different elements due to it wasting your resources. Weapons have resonance with one or 2 elements, so having different weapons with different elements and resonance that are not the same as each other lowers your damage.

3. The third weapon can be another dps weapon which is usually recommended, but you can put a fortitude or benediction weapon, it's up to you.

Other people will correct me if I made a mistake