A switch statement?

One option would be to use an enum with a label computed property.

enum TransitionState {
    case stageOne, stageTwo, stageThree

    var label: String {
        switch self {
             case .stageOne: "Stage one!"
             case .stageTwo: "Stage two!"
             case .stageThree: "Stage three!"
        }
    }
}

struct MyView: View {
    @State var transitionState: TransitionState = .stageOne

    var body: some View {
        Text(transitionState.label)
    }
}

It looks like an enum with calculated properties would be great here.