Multiple unions

x<-sort(sample(1:20, 9))
y<-sort(sample(10:30, 9))
z<-sort(sample(20:40, 9))

Both of the below produce the same output

unique(c(x,y,z))
[1]  1  2  4  6  7  8 11 15 17 14 16 18 21 23 26 28 29 20 22 25 31 32 35

union(x,union(y,z))
[1]  1  2  4  6  7  8 11 15 17 14 16 18 21 23 26 28 29 20 22 25 31 32 35

A not necessarily memory efficient paradigm that will work with GRanges is

Reduce(union, list(x, y, z))

The argument might also be a GRangesList(x, y, z) for appropriate values of x etc.