Yes, there are big differences in accent in the English spoken by natives of the three countries you mention. You could also mention other countries whose official language is English, such as India or Nigeria. *Usually most native speakers of English can understand each other.* (It is impossible to give a percentage.) Especially nowadays, when people of these countries watch television and movies that come from other English-speaking countries. If they don't understand each other, they can ask for clarification. But sometimes the "accent" of one person cannot be easily or at all understood by the other person. For example, an American (or even an English person) may not be able to understand a person from Ireland or Scotland who is speaking English. It usually depends on pronunciation of words and the amount of local idioms that the speaker is using as to whether his "accent" (a better word is "dialect") can easily be understood by other English speakers. It is the regional dialect, slang, vocabulary, and pronunciation that are the most troublesome. The grammar presents less of a problem. So, sometimes if I am watching a movie from Ireland or Scotland or England in which the actors are talking with a particularly hard dialect to understand, I may have to use subtitles to help me understand what they are saying--even though we are both native speakers!

Adding just a little to Alan's excellent answer. Britain, even England & the American have numerous accents within their boarders. With practice you can perhaps tell someone is from Texas or South Carolina, or BC from their accent & the way they speak. When I travel, some people ask whether I am from Australia, some ask from Canada. This suggests that some New Zealand accents are close to these. I suspect the main difference between British & US English is the grammar rather than the accent.

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Wait i will calculate all differences and i will say you