radius: Both the British and the American pronunciation is [ˈreɪdiəs], listen to it here. German native speakers should be careful not to pronounce the word in the German way, [‘rɑːdiʊs]. Finally, the plural of radius is radii, which is pronounced as [ˈreɪdiaɪ].
circumference: The word is to be stressed on the second syllable, not the third. Stressing it on the third syllable often leads to a pronunciation similar to the word interference. Instead, the correct pronunciation is [səˈkʌmfərəns] in British English, and [sərˈkʌmfərəns] in American English. In particular, whereas the third syllable of interference is pronounced just like the word fear, the -ference in circumference is pronounced just like -ference in the word conference. Listen to the pronunciation here.
diameter: Similar to circumference, people often stress diameter on the third instead of the second syllable. Moreover, diameter is another example where -meter is not pronounced like the unit of distance but with a short i instead, see my previous post on the subject. The British pronunciation is [daɪˈæmɪtə(r)], whereas the American pronunciation is [daɪˈæmɪtər]. Listen to it here.