Launch hashcat with conrete letters in password counts

[WhiteTiger]

Hello. For example, i has md5 hash. And i known what this hash it`s a 7-length password contains: 2 letters "a", 2 letters "b", 3 letter "c". I don`t known positions of those letters.
How can launch hashcat for fastest brute in this situation? If i run mask ?1?1?1?1?1?1?1 and --custom-charset1 abc - this will low effectively because hashcat check "aaaaaaa", "bbbbbbb" and other wrong passwords.