Mr Biden won in all three states to hold votes, expanding his lead.
Mr Sanders will discuss his options with supporters, his campaign said.
Meanwhile, Mr Biden appealed to Sanders backers to anoint him as the Democrat nominee to face President Donald Trump in November.
What are the results so far?
Mr Biden, 77, won a big victory over Mr Sanders, 78, in Florida, the biggest prize of the night with 219 delegates.
The former vice-president won 62% of the vote and about 130 delegates. Mr Sanders, with only 23%, took some 48 delegates, with the rest still to be distributed.
In Illinois, Mr Biden trounced the left-wing Vermont senator, with a margin of over 20% and at least double the number of delegates.
With most votes counted in Arizona, Mr Biden has a double-digit lead over Mr Sanders in the south-western state.
Navy Vet
Article URL : https://www.bbc.com/news/world-us-canada-51947795