does any one know why 3bb changes the ip address about once a week ???

Which package ? I've got the 5/1M Premier and it uses the same IP for from a week up to more than a month, depends. Can't remember if 3BB had a fixed IP as an extra (as in extra cost) service.

Since it is a dynamic IP from a DHCP server they will have a lease time. Common would be 24 hours lease renewal. If not connected, example router shut down for that period, during that lease expiration period it will release it to someone else and you will get another one. Also, they may periodically perform maintenance resetting the lease tables.

In my case 3BB plain drop the conn and the dhcp gives a new IP upon reconnect, happens even though there's 24/7 traffic from the multitude of smart phones, laptops and desktops we have connected to the router.

I don't know why they do that, but True was far worse, their PPPoE server went offline 3-4 times a day and when finally up, the IP could be in a totally different range. If I have to guess, the maintenance techs are of the M$ brigade, "just boot it and it'll run for a while".