First, unless you've defined your own type "Real", you need to use a
built-in type. Numeric values stored in XL cells are always of type
Double, so that's safe to use.

Second, you can't Dim a variable that you've already declared as an
argument in the function's argument list

Third, 'Set' is only used to assign an object variable to an object, for
instance, a range variable to a range as in

  Dim rng As Range
  Set rng = Workbooks("Sayaka").Sheets("Sheet1").Range("D34")

Now the variable rng can be used in place of the range object (in this
case, cell D34 of Sheet1 of workbook Sayaka).

Other (non-object) variables just use the assignment operator (=), so
for instance

  Dim Reactant As Double
  Dim Yield As Double
  Reactant = rng.Value
  Yield = rng.Value

So you might try:

  Public Function Process1( _
           ByVal Reactant As Double, _
           ByVal Yield As Double) As Double
     Dim AResult As Double
     Dim BResult As Double
     Dim CResult As Double

     AResult = (Reactant / (1 - Yield))
     BResult = (Reactant / (1 - Yield)) * 2
     CResult = (Reactant / (1 - Yield)) * 3

     Process1 = AResult + BResult + CResult
  End Function

And call the function from the worksheet as

  =Process1(Sheet1!D34, Sheet1!E34)

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