Convex union operation in R

I have 3 vectors

a = c(3,7) 
b = c(4,6)
c = c(2,6) 

I would like to make the union of these 3 sets. I could use the union() function but "convex" union requires that the vector c is removed from the union because it is dominated by a, which is higher for the two elements.

Any idea of a simple way to do it?