df1
  v1 v2 v3
1  1  2  a
2  2  3  b
3  3  4  c
4  4  5  d
5  3  5  e

df2
  v1 v2 v3
1  1  2  j
2  2  3  i
3  3  4  t
4  3  5  r

df3
  v1 v2 v3
1  2  3  t
2  2  4  g
3  6  7  i
4  8  9  t

v1 v2  v3
1  1  2 aj0
2  2  3 bit
3  2  4 00g
4  3  4 ct0
5  3  5 er0
6  4  5 d00
7  6  7 00i
8  8  9 00t

> df12 <- merge(df1, df2, by=c("v1", "v2"), all=TRUE)
> df123 <- merge(df12, df3, by=c("v1", "v2"), all=TRUE)
> df123$v3 <- paste0(df123[,3], df123[,4], df123[,5])
> df123$v3.x <- df123$v3.y <- NULL
> df123$v3 <- gsub("NA", "0", df123$v3)

dfs <- list(df1, df2, df3)
merged <- Reduce(function(x, y) merge(x, y, by=c("v1", "v2"), all=T), dfs)
v3 <- gsub("NA", "0", apply(merged[,c(-1, -2)], 1, paste, collapse=""))
data.frame(v1=merged$v1, v2=merged$v2, v3)
#   v1 v2  v3
# 1  1  2 aj0
# 2  2  3 bit
# 3  2  4 00g
# 4  3  4 ct0
# 5  3  5 er0
# 6  4  5 d00
# 7  6  7 00i
# 8  8  9 00t