What is the medical term meaning three times a day?

Ter in die is Latin for "three times a day". The abbreviation TID is used in prescriptions.
The medical abbreviation for 3 times a day is t.i.d.
TID, which stands for the Latin phrase ter in die.
T.I.D. = ter in die

T.D.S. is rare in the US, as is the term 'medicaments'

t.d.s. ter die sumendum or sumendus (latin phrase) or tid.

Means to be taken (or swallowed) three times a day. Usually this is for medicaments. Doctors prescribe them in prescription.