Ultimately, this question is arguing semantics about who counts as a Holocaust victim. Looking at the data from the United States Holocaust Memorial Museum quoted in denten's answer, we would have to include either Soviet civilians or Soviet prisoners-of-war for the figure of at least 5 million non-Jews dying in the Holocaust.
Is it reasonable to include three million Soviet prisoners-of-war? Khan Academy includes them:
The Holocaust was the deliberate killing of millions of people by Adolf Hitler's Nazi party, the German military (the Wehrmacht), and local collaborators across Europe. The victims included 6 million Jews, somewhere between 250,000 and 1 million Roma (often mischaracterized as "gypsies"), 3 million Soviet prisoners-of-war (POWs), several million non-Jewish Eastern European civilians, and hundreds of thousands of other people targeted because of their race, political affiliation, disability, religion, or sexual orientation.
As does the Canadian Encyclopedia:
The Holocaust is defined as the systematic persecution and murder of 6 million Jews and 5 million non-Jews, including Roma and Sinti, Poles, political opponents, LGBTQ people and Soviet prisoners of war (POWs), by Nazi Germany from 1933 to 1945. Jews were the only group targeted for complete destruction.
BBC Newsround (apparently kids-oriented) writes:
Who was killed or persecuted in the Holocaust?
We know that the victims included:
- Jewish people
- Roma and Sinti people ('Gypsies')
- Slavic people, especially in the Soviet Union, Poland and Yugoslavia.
- Disabled people
- Gay people
- Black people
- Jehovah's Witnesses
- Political opponents
"Why Teach the Holocaust" from Journal of Jewish Education (1981):
Also, students will become aware of other aspects of the Nazi Holocaust: the millions of
non-Jewish victims — Slavs, Gypsies, resisters — and the heroism of those, including some
Germans, who sought to protect potential victims at great personal risk
"Other Victims of the Holocaust" from Dialectical Anthropology (2000):
The Holocaust must be defined to include not just the Nazi and German atrocities against the Jews immediately before and during WWII, but also atrocities against other victims of different races and nationalities and social groups during the same period. […] A synchronic approach would help to rectify the lacunae in the literature, which has both neglected or peripheralized the Holocaust of such groups as Blacks, Gypsies, Slavs, etc. […] The holocaust atrocities against the Jews, Blacks, Gypsies, Slavs, and others were so inhuman, so atrocious and diabolic that any attempts to minimize the evils of holocaust on any group would be tantamount to betrayal and dehistoricization of events of the last century. Our entire
These aren't all scholarly sources, or meant to be a representative sampling of sources, just meant to be some examples of a broader definition that would include at least 5 million non-Jews.
Lexico defines the Holocaust as this:
the mass murder of Jewish people under the German Nazi regime during the period 1941–5. More than 6 million European Jews, as well as members of other persecuted groups such as Romani and gay people, were murdered at concentration camps such as Auschwitz.
the mass slaughter of European civilians and especially Jews by the Nazis during World War II
Dictionary.com restricts its definition to European Jews in Nazi concentration camps:
the systematic mass slaughter of European Jews in Nazi concentration camps during World War II.