Swift can a switch case statement have multiple arguments?

Yes, take a look at Swift's documentation on switch statement.

In order to achieve what you want, you need to check for the current value of myLetters:

var myCondition: Bool {
    switch self {
    case .a, .b: return true
    case .c: return false
    }
}

If you want to group cases with the same assosiated value, you can do the following:

var myCondition: Bool {
  switch self {
  case .a(let value), .b(let value): return value
  case .c(let value1, let value2): // do stuff with value1 and value 2
  }
}

Unfortunately you can't currently combine the let statements to a single one.