If Calexit comes to pass, which of the three states remain in the US? Also, will the ratification process pass? And most importantly, only one person gives final approval of the deal; the President of the United States.
Can you imagine if this all comes down to Trump having to sign off on this?
In regard to splitting California into three states, this is provided for in the Constitution, Article 4, Section 3:
“...no new State shall be formed or erected within the Jurisdiction of any other State; nor any State be formed by the Junction of two or more States, or Parts of States, without the Consent of the Legislatures of the States concerned as well as of the Congress.”
Based on this, if the California legislature votes to split into three states, all it takes is a majority vote in the House of Representatives and Senate to make it so. There is no requirement that the Executive approve it in any way.
As far as State secession goes, I do not believe it is Constitutional at all. There is no provision for it in the Constitution by design. And we had a war that decided that.