Most Belorussians and Ukrainians understand Russian due to historical reasons. I think Russians can understand a lot from written text in Ukrainian and Belorussian. But not that much from spoken language. There are some difference in vocabulary... compare it with German, Dutch and English. As for grammar and pronunciation the difference is not as great as between languages mentioned above.

The grammar of Russian and Ukrainian languages practically do not differ. For example, I being a bearer of the Russian language understand Ukrainian and Belarusian, but don't say any one of them. So my classmate moved to Ukraine, where to study in College (despite the fact that he, too, speaks in Russian). And in General, the Ukrainian language is Russian-Polish dialect.