From the NHS Cervical Screening Programme website:
Is cervical screening effective?
Whilst cervical screening cannot be 100 per cent effective, cervical screening programmes have been shown to reduce the incidence of cancer in a population of women. For example:
Percentage of Cancer Preventable
(Protection offered by a single negative smear)
20-39 40-54 55-69
3-yearly screening 41% 69% 73%
5-yearly screening 30% 63% 73%
Sasieni, Adams, and Cuzick, BJC 2003
It's thought that CIN3 (the highest PRE-cancerous change) can take between 10-15 years to develop. The screening interval has to be a cost-effective balance between picking up abnormalities which need treating and detecting minor changes which will go away on their own.
The more frequently you are screened, the more likely you are to get over-treated without picking up a significant number of those that need treating. In a three year period an individual woman is unlikely to develop cancer if their last test was negative.
Attending regularly at the appropriate intervals is the best way to go.
As zazen says, if you have symptoms - they should be checked out, either by GU or Gynae, cervical screening is not a 'catch-all' diagnostic test for everything in that area
Other tests can more appropriate depending on your symptoms, so please go back to your GP for further advice.