Margetts, 22, only signed for the Imps in July, but has since scored five goals in seven National League games.
Smallwood, 25, joined the Millers from Middlesbrough in January 2014 and played 112 games for the club.
England under-20 player Toffolo, 21, has made three appearances for Norwich.
The 21-year-old had loan spells at Peterborough and Rotherham last season, playing in seven games for each club.
Find all the latest football transfers on our dedicated page.
Scunthorpe have signed Lincoln striker Jonny Margetts on a two-year deal for an undisclosed fee, and Rotherham midfielder Richard Smallwood and Norwich defender Harry Toffolo on loan.