@ pao
those public ip addresses are just for reserved or backup as what others said. you cannot use them simultaneously. the purpose of those is for in case the ip you are currently using becomes ill.
as for that 2.4mbps instead of 3mbps, its normal. your line is not dedicated, it is shared just like mcwd pipes in your town, so its normal for it not to reach the subscribed bandwidth 100%.
this might help; enable the QoS feature in your adapter in the windows OS you have there. if your router has the QoS feature too, enable this also. this can help shape you network there to prevent single workstation to suck all the bandwidth of your connection. because even if you have a connection bigger than that 3mbps, let say 20mbps, still wont survive if one workstation shall do excessive P2P downloading and uploading.