Here are a few options, each with a formal tone, ranging from direct to analytical:

Option 1 (Direct & Formal)

Three Presidential Candidates Targeted by Foreign Disinformation: Assessing the Threat to the French Election Campaign

Option 2 (Analytical & Professional)

The Impact of Foreign Disinformation on Three Candidates: Evaluating the Risk to the French Presidential Election

Option 3 (Headline Style)

Foreign Disinformation Campaigns Target Three Candidates: A Threat Assessment for the French Election

Option 4 (Most Formal/Institutional)

An Examination of Foreign Disinformation Targeting Three Candidates and Its Implications for the Integrity of the French Electoral Process