Can't you just use leveldb scan and print all the keys and values. For example to print all keys and values in the database you can do the following :
leveldb::Iterator* it = db->NewIterator(leveldb::ReadOptions());
for (it->SeekToFirst(); it->Valid(); it->Next()) {
cout << it->key().ToString() << ": " << it->value().ToString() << endl;
}
assert(it->status().ok()); // Check for any errors found during the scan
delete it;
Now GetApproximateSizes give zero value because you have just inserted 3 keys and they are still in memory (memtable) and has not reached the filesystem yet. Once the memtable is full (by default 4MB) then it creates the first file in level 0. So it is more useful on a bigger database and a larger key range.
In your case the only place in filesystem the data is present would the redo log and if your keys are strings you could do a quick check by calling 'strings logfile' (in linux) in the db directory to print out the strings part of your keys as a quick way to confirm that data has gone in.