A lonely molar tooth does not necessarily decay unless there is any infection. As you have a decay in your second molars, We would recommend that you get this examined by an Orthodontist for evaluation as you may require antibiotics and possible root canal
therapy. Once the infection and the decay have been cured, there will be no problem in your third molar. As for whitening of teeth, you can ask your dentist
to perform a cleaning procedure where the stains on the teeth are removed. Apart from these brush your teeth regularly, know the way to brush from the dentist. Have a dental check up regularly atleast once a year though you dont have any problem assosciated with your dentition. These will detect the early problem with teeth.