Is there liability release form for landlord in case child falls out window when told not to open windows more than 3 inches?

You could, theoretically, obtain a signed release but then again that is not a guarantee to insulate you from liability. Simply, if the windows open enough to allow a child to fall out, a clever lawyer could argue that this is a dangerous condition, especially if you were aware of it. I think realistically you could have a tamper-proof window stop installed and/or screens that lock in and you'd be better off. These are cases I have been on the opposite side of and there are often defenses to liability. Keep in mind that the cases are often large due to the serious damage or death that can occur. I would contact the insurance company for your building and see what they suggest.