Clinton Sentance, 36, of Tennyson Street, Gainsborough, pleaded guilty at Lincoln Crown Court on Wednesday to two charges of robbery carried out on March 19 this year and a further charge of robbery the following day.
The robberies were at addresses in Ropery Road, Gainsborough; Portland Terrace, Gainsborough; and South Street, Morton, near Gainsborough.
Sentance also admitted three linked charges of possession of a knife.
The charges follow a series of knife-point robberies of elderly people in their own homes.
Sentance was remanded in custody and is due to return to the crown court on June 15 to be sentenced.