The Robins knew a win would guarantee promotion, and David Noble volleyed them ahead after eight minutes.
He doubled their lead just before the break, and Alex Russell sealed the result 10 minutes into the second-half. Marc Newsham pulled one back for the Millers.
In-form Blackpool will head into the play-offs with their confidence high, after a 6-3 win at Swansea. The result saw them claim third spot in the table, with Nottingham Forest dropping to fourth, while the Welsh side missed out on the play-offs.
Andy Morrell claimed four of the goals for the Seasiders, with Keigan Parker netting a brace.
Forest drew 0-0 with Crewe at the City Ground, in a result which was rendered largely meaningless by Bristol City's success.
Yeovil will meet Forest in the play-off semi-finals, a 2-0 success over Gillingham rounded off their regular season.
Oldham's 1-0 win over Chesterfield, thanks to Chris Porter's header, sealed the final play-off position.
A first-half hat-trick from Craig Curran gave Tranmere a 3-1 win over bottom club Brentford.
Doncaster and Northampton played out a 2-2 draw, as did Bradford and Millwall.
Port Vale were 2-1 winners over Bournemouth, and Huddersfield beat Leyton Orient 3-1.
John Finnigan converted a late penalty for Cheltenham to rescue a point in a 1-1 stalemate with Brighton.