Sally-Ann Hart, who is standing as a Tory candidate in Amber Rudd's former Hastings and Rye seat, shared a video in 2017 which implied Jewish philanthropist George Soros controls the EU.
St Helens South and Whiston candidate Richard Short questioned in a tweet in 2013 whether journalist Melanie Phillips's allegiance was to the UK or Israel.
And Lee Anderson, who is running in Ashfield and Eastwood, is said to be an active member of a Facebook group where Soros conspiracies are promoted.
A Conservative Party spokeswoman said: "These matters are being investigated.
"Discrimination or abuse of any kind is wrong, and the Conservative Party takes decisive action to deal with any incidents of hatred, abuse or intimidation.
"We are committed to stamping out the scourge of anti-Semitism in our society and supporting our Jewish community.
"Our complaints process is rightly a confidential one but there are a wide range of sanctions to challenge and change behaviour, including conditions to undertake training, periods of suspension and expulsion, and these are applied on a case-by-case basis."
Andrew Gwynne, Labour's national campaign co-ordinator, said: "Johnson must answer for the anti-Semitism being promoted in his name.
"One Conservative candidate 'liked' a Nazi slogan, another candidate promotes a group named after Johnson which peddles anti-Semitic Soros conspiracies, and a third candidate accused British Jews of being more loyal to Israel than to the UK, a classic anti-Semitic trope.
"Boris Johnson said members who make racist comments are 'out first bounce'. So why is he refusing to suspend these three candidates, none of whom appear to have apologised?
"Johnson has never called out and condemned anti-Semitic Soros narratives among his supporters.
"On the contrary, the Conservatives whipped their MEPs to vote in support of the Hungarian government which peddles the Soros conspiracy and appointed a senior government adviser who promotes this narrative.
"Anti-Semitism is clearly rife in the Conservative Party from top to bottom."