I can't absolutely confirm when these players can be traded but Spotrac says Richardson on 7 Jan 2022, Smart on 17 Jan 2022, and JHern, no restriction is listed.
As to the trade, I think it is too much of a "blow up" to make at this time. Smart and Richardson are core players. The team has been largely disappointing in 5 games so far but I am not ready to go to this extreme. You don't "shake up" the team after just 5 games. Hield and Bagley are nice players but they are probably both bench players for us. It would be great if Bagley became a starting level PF (he is not one right now) but that would still leave Hield coming off the bench.
Hield and Richardson are probably nearly a wash when defense and everything is figured in and so you are giving up Smart for a questionable big with a lot of potential but who can't even get on the court in SAC. It would shake things up, that is for sure but I am not sure it makes us better.