> The Cal 3 proposal actually made some sense
Not really, even on the scale of “crazy proposals to divide California into multiple states” , of which there have been a couple hundred since it was admitted to the union.
> it would have given the people living there more representation in congress in the form of 4 new senators, as well as more local control
Sure, and Cal58 (each existing county as its own state) would give them 114 new senators, and even more local control, by the same standards. Cal3 retained the main “problem” most recent “split California” proposals aim to fix, the perceived representation problem (particularly in the statewide executive and U.S. Senate) resulting from the deepest red parts of the State (which are on par with some of the deepest red parts of the country) being diluted in a big blue state. Two of the three Cal3 states (NorCal, Cal) would have been solid blue, including the one with the reddest parts of the existing State (Cal), and the other (SoCal) would have (at least, from the political state at the time) been fairly competitive (though also pretty solidly blue, in Presidential terms, if 2016 and 2020 worked out the same way by county.)
It also would have turned a lot of broad issues in the State, which cross the proposed borders, into potential federal issues, reducing local control (and also reducing the possibility of any favorable action in the presence of federal hostility – common given the structural rightward tilt of federal representation [which, while it would help a tiny bit with, it wouldn't neutralize] – or deadlock.)
> You would think that there would be broad support for the proposal considering a lot of people in these large left-leaning states often complain that their vote is not “worth as much” as a vote in a less populated state.
You would think that if you were thinking shallowly about that one complaint, and not the whole host of issues involved in splitting up an existing state.