you have-3.00 no that means you have myopia
myopia is of various grades.
you are having moderate myopia.
myopia can increase with age but it will stabilizes when you reach age of 21years.
you are not underweight
and it is not the cause of your number
i would advise you to visit an ophthalmologist every 6months to 1 year and get your refraction
dont worry there is no problem,just wear your glasses regularly.
hope this helps