Steel Magic — Andre Norton

A girl and her two brothers are transported back to the time of King Arthur, where they must return three magic talismans to their rightful owners or remain trapped forever in the distant past

The adventure begins when Sara Lowry wins a picnic basket at the Firemen's Strawberry Festival. She and her brothers, Greg and Eric, are staying with their uncle at his Hudson Valley estate while their parents are in Japan. Fascinated by the haunting history of the old manor, they pack a picnic lunch and begin their search for the legendary lost lake. They discover a medieval castle -- and suddenly they're enveloped by a gray mist that transports them back to the time of King Arthur! There, they are given an urgent mission to recover three lost talismans: Arthur's sword, Excalibur; Merlin's ring; and the horn of Huon, Warden of the West.
 
Can Sara, Greg, and Eric fulfill their quest and save Avalon -- the only place that stands between the Powers of Darkness and the mortal realm -- and return to their own time?