[R] a statistic question,a bit off-topic,but important

[ronggui]

it seems that all the existing prop test assume 2 independent or matched sample.but in the real world, many situations are not as we assume.for example,i do a research on the voter's prefernce through a random sampling.and the sample shows that 23% of the sample choose A,28% choose B,the others choose C.and i want to test the diference between the proportion choosing A and B (23%-28%=-5%) is sinificant or just due to sampling error. i think the prop.test is not proper here. and it seems all the stat textbooks do not deal with these problem.
is it a igorance in the statistical world or just something else?