It's not secure at all. If someone knows the rules of the system, the entropy on that is tiny, because it's basically a 2 letter password with only 6/3 options.
The only security would be from the obscurity of the attacker not knowing the password rules.
That was obviously an oversimplified example to explain the rules.
In practice you could make it as obscure as you want, while keeping it easy for you to remember.
Like the sentences I just typed here. No limit on the number of characters. I could enter different long sentences each time, as long as the characters at specific positions match certain sets.
You're not getting anywhere, because people trying to guess your password don't have to guess your scheme. All you're doing is making it easier for them. There is no sense in which you are making it harder.
In the optimum case, you'd require them to get the right characters in the slots you're counting, but to not use the wrong characters in the slots you're not counting, thus demonstrating that they actually know the scheme in question and aren't just getting lucky. There would be exactly one character you'd accept in the slot you're counting, and there would be exactly one character they could use to indicate they understand your pattern in the slots you're not counting. This maximizes the chance they have proved to be in possession of your password, rather than just getting lucky because you didn't count their misses. This is, of course, simply using a password normally.
If only that made it as secure as you want.