You do not need a sickness certificate if you are off work for three days. Your employer should provide you with the form for "Self Certification" of sickness or you can do it electronically here Self Certify Sickness Form
Please do not ask for sickness certificates for less than one week, unless your employer is prepared to pay a fee.
The doctors can provide a doctor's sick note for patients they have seen who are off work for a week or more and requesting a continuation of a certificate. You can request this by submitting an online form..
If this is a new request for a sick certificate for 7 days or more you will need to see a GP in a routine appointment to discuss.
www.gov.uk/taking-sick-leave
www.gov.uk/government/publications/statutory-sick-pay-employees-statement-of-sickness-sc2