It's probably important to establish first that, from what little English-language scholarship there is on the matter, scholars don't generally regard the regime of Boris II in Bulgaria as a fascist one. Authoritarian, yes, but fascist, no. Interwar fascism in Bulgaria was disparate and divided between many different groups, who were less radical than some of their other European counterparts, and who failed to have any real influence on politics owing to their lack of agreement on a clear vision for the country. It was only really the Marxist anti-fascists of the period that regarded Boris as a fascist. That said, it has been suggested that Bulgarian fascism a movement was influenced by Italian fascism, which is a theme that you will see throughout this answer.
The Arrow Cross regime (1944-45), or the 'Government of National Unity', was plainly and simply a puppet government established under Nazi occupation. Germany invaded the country in October 1944, deposed Miklós Horthy (who had led the country as 'consort' since 1920) and installed Ferenc Szálasi as leader. Horthy himself is not generally regarded as having been a fascist, though he was conservative and ruled with almost-absolute power. He also had a prime minister by the name of Gyula Gömbös, who was nicknamed 'Gombolini' because he was inspired by and styled himself after Mussolini. He was never able to implement anywhere near the programme that the Duce did, though. That the previous leader was not a fascist is not to say that fascism in Hungary came from nowhere in 1944. Like in many European countries, the Great Depression had knocked the faith of many people in the predominant capitalist system. Along with admiration for the corporatist state introduced in Italy, being surrounded by other countries with growing fascist movements, and the perception of fascist strength in the success of the Francoist forces in the Spanish Civil War, many Hungarians began to view fascism as a viable alternative to democracy in the late 1930s and early 1940s. So, there was an 'organic' element to the regime in that it probably enjoyed some support from the people, although it was quite suddenly imposed on them by an invading force.
The Iron Guard regime (1940-41) was perhaps the most 'organic' of the regimes you have mentioned. It was the foremost fascist movement in Romania before coming to power, having been established in 1927 and slowly grown in stature and influence since then. Though it was influenced by other fascist movements in Europe, it included a rather unique theological aspect, publicly declaring that it sought not just the rebirth of the Romanian nation, but Christian life itself. The regime came to power because, by 1940, King Carol had lost almost all political standing. He had agreed to cede northern Transylvania to Hungary in August of that year, which angered a population which was still hurt from having lost territory it viewed as its own at the end of the First World War. There was an attempted coup on 3 September, which failed, but led Carol to believe that he had no choice but to essentially hand power to the Legion, which he did three days later. He did not actually appoint the leader of the Guard at the time, Ion I. Moţa, but Ion Antonescu, who had close ties to them. This happened independently of the Nazis, though Moţa would later agitate for closer ties to Germany. This was in spite of the fact that Germany had showed little interest in the Guard for most of its history. Moţa was inspired by their anti-semitism, and celebrated the Nazi coming to power, but the feeling was not mutual. Likewise, he admired the Italians, but they were suspicious of his staunch anti-semitism, which was not a major feature of Italian fascism until the late 1930s.
Sources used:
Raul Cârstocea, 'Native Fascists, Transnational Anti-Semites: The International Activity of Legionary Leader Ion I. Moţa', in Arnd Bauerkämper & Grzegorz Rossoliński-Liebe, Fascism without Borders: Transnational Connections and Cooperation between Movements and Regimes in Europe from 1918 to 1945 (New York: Berghahn Books, 2019), pp. 216-242.
Deborah S. Cornelius, Hungary in World War II: Caught in the Cauldron (New York: Fordham University Press, 2011).
James Frusetta & Anca Glont, 'Interwar fascism and the post-1989 radical right: Ideology, opportunism and historical legacy in Bulgaria and Romania', Communist and Post Communist Studies 42 (2009), pp. 551-571.
Grzegorz Rossoliński-Liebe, 'Inter-Fascist Conflicts in East Central Europe: The Nazis, the “Austrofascists,” the Iron Guard, and the Organization of Ukrainian Nationalists', in Arnd Bauerkämper & himself (eds.), Fascism without Borders: Transnational Connections and Cooperation between Movements and Regimes in Europe from 1918 to 1945 (New York: Berghahn Books, 2019), pp. 168-191.
Edit(s): Proof reading.