Userform: ComboBox Automatically Populating

That's what I was just attemping. However, when I try your code I get an error

"Run-Time Error '1004'

The Sort reference is not valid. Make Sure that it's within the data you want to sort, and the first sort by box isn't the same or blank" Specifically the code I used was

Sheet3.Range("A1").CurrentRegion.Sort Key1:=Range("A1"), Order1:=xlAscending, Header:=xlGuess, _
        OrderCustom:=1, MatchCase:=False, Orientation:=xlTopToBottom, _
        DataOption1:=xlSortNormal