Knowing how to calculate the three days when served with a three day notice is important and it is good that you asked.  Generally speaking, the three days begins the next day after the notice has been properly served (another very important issue) on you as the tenant.  If the third day falls on a Saturday or Sunday or even a legal holiday, then the notice is extended to the following business day.  Now, if you do not pay or quit then the landlord must start a summary proceeding to evict you.  But if you just need to know so that yo can have more time to get the money together then there you go.  Good luck.