How to use multiple measures with var syntax in a SWITCH()?

darkmetrics Maybe:

My Measure = 

var PrevMonthYear = IF(MONTH(TODAY()) == 1, YEAR(TODAY())-1, YEAR(TODAY()))
var PrevMonth = IF(MONTH(TODAY()) == 1, 12, MONTH(TODAY()) - 1)
var PreviousMonthActiveSales :=
CALCULATE(
SUMX(FILTER('Sales', 
            YEAR('Sales'[Date]) = PrevMonthYear &&
            MONTH('Sales'[Date]) = PrevMonth
            ),
    'Sales'[ActiveSales]),
    ALL('Sales'[Date])
)

var PrevQuarterYear = IF(QUARTER(TODAY()) == 1, YEAR(TODAY())-1, YEAR(TODAY()))
var PrevQuarter = IF(QUARTER(TODAY()) == 1, 4, QUARTER(TODAY()) - 1)

var PreviousQuarterActiveSales := 
CALCULATE(
SUMX(FILTER('Sales', 
            YEAR('Sales'[Date]) = PrevQuarterYear &&
            QUARTER('Sales'[Date]) = PrevQuarter
            ),
    'Sales'[ActiveSales]),
    ALL('Sales'[Date])
)

var PrevYear = YEAR(TODAY()) - 1

var PreviousYearActiveSales := 
CALCULATE(
SUMX(FILTER('Sales', 
            YEAR('Sales'[Date]) = PrevYear
            ),
    'Sales'[ActiveSales]),
    ALL('Sales'[Date])
)

FirstTablePrevPeriodSales = 
SWITCH
(
    SELECTEDVALUE(DebitSalesDateSlicer[Period]),
    "MTD", PreviousMonthActiveSales,
    "QTD", PreviousQuarterActiveSales,
    "YTD", PreviousYearActiveSales
)

RETURN
FirstTablePrevPeriodSales