Do "every three days " and "every third day" mean the same thing?
For example:
A. I see him every three days.
B. I see him every third day.
Let's say, I see him today(Friday). Considering every three days/every third day, do they both mean I will see him again on Monday?

Question

Do "every three days " and "every third day" mean the same thing?
For example:
A. I see him every three days. 
B. I see him every third day. 
Let's say, I see him today(Friday). Considering every three days/every third day, do they both mean I will see him again on Monday?

Answer

"Every three days" is the more common way of saying it. 
"Every third day" is perfectly natural, but because it is less common it sounds more polished to me. You might see "every third day" more often in written English, for example.

Answer

every three days=every third day.

Answer

They mean the same thing, at least in American English. Yes, it would mean Monday.