It’s probably safe from the casual attacker who just downloads a password list and runs a one word dictionary attack, but for a dedicated attacker, let alone a nation state, it’s not secure.
TL;DR: Just use AES. Even an ASIC isn’t powerful enough for that. Searching the entire key space would take more energy than the universe has. Compare that to DES that can have its entire key space searched in a few days.
Edit: you said triple DES, not single. My point still stands. DES, even 3DES, is not secure. If I can crack a DES password in 4 days, I can crack a 3DES password in 12. AES with a strong password is virtually uncrackable.
It's multiplicative, not additive. 3DES is about 2^56 times as difficult to crack as DES. (Not 2^112 times because there is an attack that effectively limits it to twice the effective bits of DES, rather than the three times you might expect at first).
* Meet-in-the-Middle attack.
This attack is surprisingly simple, if you encrypt the message twice by
ciphertext = encrypt(encrypt(message, key1), key2)
decrypt(ciphertext, key2) == encrypt(message, key1)
But in this case, the attacker can obtain all the 2^56 possible encryption of message by enumerating key1, put it in a lookup table (assume the table-lookup time is O(1)) , then we can try all possible decryption of ciphertext by enumerating key2. Then compare it with the lookup-table for a match, bingo!
If key is 56-bit, the attacker gets 2^56 outputs for the left side, 2^56 outputs for the right side, total number of operations is 2 x 2^56 == 2^57, not 2^112.
To increase the security claim to 2^112, we need triple encryption, not double encryption, thus 2DES is never used.
The idea that simple double-encryption doesn't work because of such a simple attack shocked a lot of newcomers.
If you’re using 3 different keys, yes, that makes sense. But if you’re just keystretching one key, wouldn’t it just take 3 times as long because you encrypt, decrypt, encrypt (3 processes)?