The Wilmington News Journal reported Sunday that the three include a start-up developing a plug-in hybrid and an Indian automaker.
The newspaper reported the Boxwood Road plant was also toured by the Penske Automotive Group, which had planned to buy the Saturn brand from GM before the deal fell though.
Two auto assembly plants have closed in Delaware in the last year.
The University of Delaware is the only known bidder for Chrysler's Newark assembly plant, which closed in December, proposing a technology park and possibly a medical campus.
Information from: The News Journal of Wilmington, Del., http://www.delawareonline.com