tell me the total magnesium sulfate dose that patient 11426 has been prescribed until 36 months ago?
select sum(prescriptions.dose_val_rx) from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 11426 ) and prescriptions.drug = 'magnesium sulfate' and datetime(prescriptions.startdate) <= datetime(current_time,'-36 month')