The times that are haraam for sex are during the woman's menstrual cycle and when one is observing a wajib fast. Other than that it's not haraam. In regards to makrooh times, it is said the first, middle and end of the lunar calendar are makrooh. From a Islamic laws perspective, having sex 2 or 3 times is not haraam but in regards to your question about it being harmful, that is something for science to answer and also depends on both the husband and wife. Using protection is not haraam.
Below are two books you can refer to for in-depth information: