Deborah Meaden first joined the British television programme Dragon's Den in series 3.
Meaden became one of the programme's most recognisable investors after joining the panel in its third series. She brought experience from family entertainment, leisure, and other businesses, and her investment decisions became a recurring part of the show's later history.
The distinction between series and calendar year is important. Dragon's Den series are broadcast seasons, and the number identifies Meaden's position in the programme's run rather than a standalone year. She was not part of the original five-person panel seen in series 1.
Viewers sometimes place her arrival in series 2 because she became associated with the show early in its life. The specific series credit is series 3, which makes that the single canonical answer.