Assuming Sam wins both her singles (not a sure thing, but fairly likely) the 3rd point could be hard to come by.
There really is no obvious choice for the second singles player. Jarka usually lifts her game for FC, but she's a mess atm, and hasn't got a great record against the Swiss anyway. But you wouldn't want to bet your house on either Casey or Ash winning a singles rubber either.
The best hope might be that Casey and Ash can pull off the doubles, but if the tie goes down to the wire that puts a lot of pressure on them.