If all 3 radios are on the same vessel (and the handheld will primarily be used from the same vessel), then they should all have the same MMSI #.
Do radios in the new boat have the same MMSI # as the vessel you sold?
If so, you need to cancel that (which you should do in any event) on the boat US website and get a new # for your new Vessel (to put in all 3 radios).
I replaced my radio this year (instaling this week) with one that does Polling as well as transmit DSC and could not remove the DSC from the old radio. I was told to cancel my old MMSI account # and create a new one for the new radio.
Whoever gets my old radio will have to put in their own MMSI# as their is no way to remove them.
Also, most radios let you change the MMSI once or twice (but not delete, only change to different #), then you have to send it back to factory to have them wipe out the old #'s.
Bottom line is if you sell a vessel or old radio with DSC programmed, you need to cancel your account and get a new MMSI for your new radio and/or vessel