To be fair, back when Meghan was in the UK and Queen Elizabeth II was still alive, Meghan was not #3, but more likely #6 in the order of precedence, as she was outranked by the late Queen herself and by Camilla, Sophie, Anne, and Kate. Had she stayed in the UK though, it is correct that she would now rank #3 in precedence below only Queen Camilla and the Princess of Wales.
Being #3, however, or much less #6, is not enough for Meghan, and that is why she left. She strikes me as someone who won't take anything other than #1, which is how she feels like now in her own personal realm in Montecito. In reality, however, as you said, she is only one among many influencers or celebrities in California or the USA at large and, by far, is neither the most influential, nor the most valuable one.
In any case, a few media outlets that cover US entertainment news have reported that Netflix claims to be very confident about Meghan's new cooking/lifestyle show. I wouldn't read too much into it as TV /streaming executives often say that before a show is released (it is bad publicity to talk down your own content when it has not been made public yet), but I guess we will see in two weeks. I feel like this show might be a "make-or-break" moment in Meghan's post-UK career.