The estimated saving is £30m per year. A large part of that sum will probably come from the BBC no longer having to pay the transmitter owners (Arqiva) for broadcasting the channel via terrestrial TV, with similar savings coming about through not having to pay Astra for satellite access.
BBC Three was transmitted on Freeview via the PSB1 multiplex (= 'bundle of channels')
https://ukfree.tv/transmitters/multiplexes
The spare capacity could, for example, be used to increase the number of local BBC radio stations using that multiplex but, given that the whole point of removing BBC Three was to save money, it seems unlikely that any additional BBC programming (of either TV or radio) will be added to the multiplex.