Your input appears to be a sequence of Python objects; it certainly is not valid a JSON document.

If you have a list of Python dictionaries, then all you have to do is dump each entry into a file separately, followed by a newline:

import json

with open('output.jsonl', 'w') as outfile:
    for entry in JSON_file:
        json.dump(entry, outfile)
        outfile.write('\n')

The default configuration for the json module is to output JSON without newlines embedded.

Assuming your A, B and C names are really strings, that would produce:

{"index": 1, "met": "1043205", "no": "A"}
{"index": 2, "met": "000031043206", "no": "B"}
{"index": 3, "met": "0031043207", "no": "C"}

If you started with a JSON document containing a list of entries, just parse that document first with json.load()/json.loads().

Your question is a little unclear. If you're generating hostDict in a loop:

with open('data.txt', 'a') as outfile:
    for hostDict in ....:
        json.dump(hostDict, outfile)
        outfile.write('\n')

If you mean you want each variable within hostDict to be on a new line:

with open('data.txt', 'a') as outfile:
    json.dump(hostDict, outfile, indent=2)

When the indent keyword argument is set it automatically adds newlines.

You ran into the pitfall of neglecting the fact that the \ character is also an escape sequence character in Python. Try printing out the last example instead of calling json.loads:

>>> print('{"mystring": "Line 1\nLine 2"}')
{"mystring": "Line 1
Line 2"}

No way the above is valid JSON. What if the \ character is correctly encoded?

>>> print('{"mystring": "Line 1\\nLine 2"}')
{"mystring": "Line 1\nLine 2"}

Much better, you can then:

>>> json.loads('{"mystring": "Line 1\\nLine 2"}')
{'mystring': 'Line 1\nLine 2'}

Alternatively, if you really appreciate being able to copy some text from some other buffer and paste it into your live interpreter to do decode, you may consider using the raw modifier for your string:

>>> print(r'{"mystring": "Line 1\nLine 2"}')
{"mystring": "Line 1\nLine 2"}
>>> json.loads(r'{"mystring": "Line 1\nLine 2"}')
{'mystring': 'Line 1\nLine 2'}

See that the \ is no longer automatically escaping with the newline.

Also see: How do I handle newlines in JSON? and note how this is not a problem that exists strictly within Python.

If you want to produce valid JSON file you need to write all values at once, not one value at a time (which will produce ndjson or JSON lines)

So, for a valid JSON

values = [{"first_name": "John", "last_name": "Smith", "food": "corn"},
          {"first_name": "Jane", "last_name": "Doe", "food": "soup"}]

import json
with open('some_file.json', 'w') as f:
    json.dump(values, f, indent=4)

some_file.json

[
    {
        "first_name": "John",
        "last_name": "Smith",
        "food": "corn"
    },
    {
        "first_name": "Jane",
        "last_name": "Doe",
        "food": "soup"
    }
]

if you really need ndjson - you can use ndjson package (need install via pip from PyPi).

import ndjson

with open('some_file2.ndjson', 'w') as f:
    ndjson.dump(values, f)

some_file2.ndjson

{"first_name": "John", "last_name": "Smith", "food": "corn"}
{"first_name": "Jane", "last_name": "Doe", "food": "soup"}

Alternative to ndjson package is jsonlines package