It means that they'll have nine hours of meetings, spread out over the course of three days. For example, they'll meet for three hours on Monday, three hours on Tuesday and three hours on Wednesday.

It means that the two (or more) people will have 9 hours to meet over the period (length of time) of 3 days. That is, 9 hours within 3 days. Or, out of 3 days time, they will have 9 hours of time together. It also implies that the 9 hours is spread out over the 3 days (for example, 3 hours each day). Or, mathematically speaking, they will be together for 9 out of 72 (3x24) hours.

During this time interval (3 days) they will have 9 hours for meeting.

thank you so much for kind answers!!! :)