Is the time period when this happens predictable?
I doubt this is related to your website. Possibly it could be related to your server, if it is doing other bandwidth intensive activities during the time period in question, or something else on the network is using all of the available bandwidth during that time. For example, many computers on a corporate network may be scheduled to download software or Antivirus updates at the same time each day, which could cause a noticeable slow down of the network during that time. Maybe some backup process that happens periodically but takes 3 hours to complete the data transfer uses lots of bandwidth and slows everything else down while it's running.
If you can set up a network monitor on your server or have access to the switch it's connected to, you can see if the total server bandwidth is much higher during the time in question. If it's not higher on the server or the server's switch port, then work outwards to the immediate network on the switch the server is connected to (which likely means a call to your hosting provider).
Lastly, the fact that the traffic increases after the period is over, suggests that intentional throttling is unlikely, since it would be an odd algorithm to stop the traffice for 3 hours and then let it increase again after that.