As far as I understand, the 5nm process will yield lower power consumption[1]. As the i3 is on the lower end of power consumption this may be enough to make it viable for applications that it would not be viable for otherwise. While their powerful CPUs are fast enough (if you look at competitors) and the power requirements there is already so high that the reduced power consumption from the 5nm process won't make them that much more attractive.
Actually there are i9 chips with same TDP as i3 [2]. I would think that the i3 still sells more units and is lower cost, so maybe a lower power i3 will be more attractive to OEMs than a lower power i7 or i9.
Not sure though, just guessing.
[1]: https://en.wikipedia.org/wiki/5_nm_process#Commercialization
EDIT:
Actually there are i9 chips with same TDP as i3 [2]. I would think that the i3 still sells more units and is lower cost, so maybe a lower power i3 will be more attractive to OEMs than a lower power i7 or i9.
[2]: https://en.wikipedia.org/wiki/Comet_Lake_(microprocessor)