I want to write a for loop that takes the value of each cell, passes it into a function called FixID(), then outputs the result of the function into the value of the cell. The below code passes in values to the function fine, but it does not write the new value to the cell.
Sub test()
Total_Rows = Cells(Rows.Count, 3).End(xlUp).Row
For Each C In Range("A3", "C" & Total_Rows )
C = FixID(C.Value)
Next
End Sub
UPDATE: Here is a quick little example I put together to test the technique, and it worked as expected...
Option Explicit
Sub Test()
Dim MyRange As Range, Cell As Range
Dim Temp As String
Dim Total_Rows As Long
Total_Rows = Cells(Rows.Count, 3).End(xlUp).Row
Set MyRange = Range("A1:C" & Total_Rows)
For Each Cell In MyRange
Cell.Value = AddShrimp(Cell.Value)
Next Cell
End Sub
'Bubba Gump Function
Public Function AddShrimp(InputValue As String) As String
AddShrimp = InputValue & " Shrimp"
End Function
Before: 
After: 
ORIGINAL RESPONSE: Make sure you define the Range you're going to iterate through like this:
For Each C In Range("A3:C" & Total_Rows)
C.Value = FixID(C.Value)
Next C
I would probably add Option Explicit to the top of your routine as well, and declare all your variables up-front too (to prevent confusion, typo-related errors etc.):
Option Explicit
Sub Test()
Dim Total_Rows As Long
Dim C As Range, TotalRange As Range
Total_Rows = Cells(Rows.Count, 3).End(xlUp).Row
Set TotalRange = Range("A3:C" & Total_Rows)
For Each C In TotalRange
C.Value = FixID(C.Value)
Next C
End Sub
AddShrimp function and it iterated right through without issue. (See my updated response above.)FixID function?c = FixID(c.value) . It needed to be c.Value = FixID(c.value) . I agree that defining variables is good practice though. Thanks for the help!
FixID?