Like noted, it helps to define what exactly you mean by "Russian." But for the sake of giving an answer, I'll go with the definition of sharing a similar identity to the people in Russia. Under this, both Belarus and Ukraine are somewhat recent converts to having a Russian emphasis, though this is far more complicated than that.

Belarus was traditionally well within the Grand Duchy of Lithuania and then the Polish-Lithuanian Commonwealth. It was actually one of the more central regions of Lithuania, though most of the people were not Lithuanian. That said they were not considered Belarusian, as the idea of a separate Belarusian identity only really started around the 1890s, long after the region had been annexed into the Russian Empire. The Belarusian language, which was again only really defined as a separate language around that time, was largely seen as a peasant dialect of Russian, though the people were not considered Russian; however they were also not seen as Lithuanian, but merely a Slavic group that was similar to the Russians, Poles, and Ukrainians.

Ukraine is also a confusing case. Traditionally the territory of Ukraine had been divided between west and east, known as the Right Bank and Left Bank, respectively (in reference to the Dnieper River that largely cuts the country in half). The Left Bank was more heavily influence by the Russians, while the Right saw greater Polish interaction (the legacy of this division is still evident today).

Again a separate Ukrainian identity was somewhat late to develop, though there are claims that the 1648 Khmelnytsky Uprising was based on a distinct Ukrainian identity, but this is not a fully justified claim. The Ukrainians recognised that they were not Russian or Polish, though similar, and though they were under separate rulers for centuries largely shared a similar identity, though the concept of a unique Ukrainian identity was far stronger in the Right Bank (Polish side).

It was really only within the last century that both regions were made more "Russian," though this also coincided with a surge in their own national identity. They had close association with Russia historically (seeing how they have similar languages, religions, somewhat close cultures), but they were never considered Russian by any means.

Define "Russian". Russian as invented by Muscovy or Russian as of descendants of Kievan Rus?