#put all data frames into list
df_list <- list(df1, df2, df3)
#merge all data frames in list
Reduce(function(x, y) merge(x, y, all=TRUE), df_list)
#for merging coloumns
merge(a,b...)
#for merging rows
(rbind(a,b...)
merge(data.frame(A, row.names=NULL), data.frame(B, row.names=NULL),
by = 0, all = TRUE)[-1]