If you are requested to submit 3 LORs and you WILL submit only 3 LORs, there is no reason why you should write that you will submit 4 LORs on your application. It may confuse adcoms. Perhaps they will be waiting for that fourth LOR to arrive. If you indicate only three recommenders and your fourth recommender sends his/her letter in the end, it will not be a problem. But if you indicate four recommenders and one won't send a letter, it can bit a (little) problem. Why do you want to sumbit 4 letters for programs that request only 3 anyway?