May I add one more thing. Sometimes "mikka" means 3 days (period) without "-kan" when associated with proper particles or verbs, such as: "kono report wo mikka de shiagemasu" I will finish this report in 3 days. I think the term "mikka-kan" often means a consecutive period, as appearing in the following expression: "Mikka-kan nani mo tabete inai" I have not eaten anything for 3 days.

Mikka usually means the 3rd day (of a month), for example, shichi-gatu mikka = July 3rd. If you would like to say 3 days (in terms of a period), we use "mikka-kan."