Name1 | Zip_code | Data
----- | -------- | ----
George|  123     |  abc
----- | -------- | ----
Marge |  456     |  def
----- | -------- | ----
Mike  |  789     |  foo

Name  |  data    | zip_code
----- | -------- | --------
Mike  | klm      | 789
----- | -------- | --------
George| xxx      | 123
----- | -------- | --------
Marge | yyy      | 456
----- | -------- | --------
Bob   | zzz      | 678

Data  |  Name    | zip_code
----- | -------- | --------
zzz   |  Bob     | 678
----- | -------- | --------
ggg   | Mike     | 789

Name1 |  Zip_code | Data | row_df2 | row_df3
----- | --------- | ---- | ------- | -------
George| 123       | abc  | 2       | NA
----- | --------- | ---- | ------- | -------
Marge | 456       | def  | 3       | NA
----- | --------- | ---- | ------- | -------
Mike  | 789       | foo  | 1       | 2

Name  | data  | zip_code  | row_df3
----- | ----- | --------- | -------
Mike  | klm   | 789       | 2
----- | ----- | --------- | -------
George| xxx   | 123       | NA
----- | ----- | --------- | -------
Marge | yyy   | 456       | NA
----- | ----- | --------- | -------
Bob   | zzz   | 678       | 1

dflist[[1]] <- df1
dflist[[2]] <- df2
dflist[[3]] <- df3

criterialist[[1]] <- c(1,2)
criterialist[[2]] <- c(1,3)
criterialist[[3]] <- c(2,3)

cross_checker <- function(dflist, criterialist){

# Insert an index column indicating the row number to be returned:
for (i in 2:length(dflist)){
dflist[[i]]$index <- 1:nrow(dflist[[i]])
}

# Next we loop over the dataframes with two for-loops:
for (i in 1:length(dflist)-1){
  for (j in 2:length(dflist)){
  dflist[[i]][,ncol(dflist[[i]])+1] <- merge(dflist[[i]], dflist[[j]], by.x=criterialist[[i]], by.y=criterialist[[j]], all.x=TRUE)$index
}
}