[R] Regression of categorical data

[Walter Anderson]

I have a set of survey data where I have answers to identify preference 
of three categories using three questions

1) a or b?
2) b or c?
3) a or c?


and want to obtain weights for each of the preferences

something like X(a) + Y(b) + Z(c) = 100%

I am at a loss how how to calculate this from the data.  Any help would 
be appreciated!