Can you be visited by more than one Noble in Splendor?
Yes, you can, but not on the same turn.

From the rules:


  At the end of their turn, each player checks the noble tiles in
  order to determine if they’re receiving a visit from one of them.
  A player can be visited if they have (at least) the quantity and
  type of bonuses indicated on the noble tile.


So all that matters is that you have at least 3 diamonds for a noble that requires 3 diamonds; there's no requirement to have an additional 3 for an additional noble.

And the rule limiting it to 1 noble per turn:


  If a player has enough bonuses to be visited by more than one
  noble at the end of their turn, that player chooses the noble to
  be received.