What Is Difference Between 1x300 Mg Eption And 3x100 Mg Eption?

HI Doc, about six months i was operated for oligodendroglioma with 100%resection the medication prescribed was eption 100 mg x 3 times daily with frisium 1 tab .10mg at bed time. recently i went for a review & was advised to take eption esr 300mg 1 tab during bed time & firisium was reduced to .5 mg daily at bed time. my question is why the doc has given 300 esr mg once, while the effect is same as 100 x 3 times? what are is side effects.