The term 'SUIT UP IN' can be solved by the three letter word DON. The term 'suit up in' is an idiom which means to dress formally, usually for a special occasion or event. It is often used in the context of wearing a suit and tie. The word 'DON' is derived from the Latin word 'donare' which means to give or present. Therefore, the phrase 'suit up in' can be interpreted as dressing formally in order to present oneself in a professional manner.