Having final grades for the admissions committees to review during your admission evaluation does matter. For majors with a smaller number of prerequisites, like Econ, it matters even more so.
It’s recommend “pretty much required” to have at least one of the introductory economics courses completed in the Fall since Spring grades will not be completed in time for your admissions evaluation. It’s also practically required to have the Calc I equivalent completed by the end of Fall, since you can’t transfer Calc II without a prior Calc I equivalent and there are currently no viable short session options for a Calculus I equivalent.
As alluded to above, they will see your Fall grades via your self-reported [Transfer Academic Update in January.](http://admission.universityofcalifornia.edu/how-to-apply/after-you-apply/update-transfer-application/)
It may. In the case you do, the committee would see that you loaded up in the Fall but would question why you didn’t take some of the courses earlier so you would have to explain your rationale of a late major change within your essays.
Regardless, it’s definitely not going to weigh on admissions to the extent that GPA and eventual course completion will so if loading up means a higher chance of a GPA drop, then it’s probably not worth it. Unfortunately a 3.68 is already on the lower end of the spectrum for Econ admits at the three of the schools you listed so grades should probably take priority. (Though I understand it’s only one “C” weighing you down so far.)
Despite bad profs, most here will agree that spreading some courses over to Summer would be easier than loading up in the Fall.
Additionally, if you are planning to TAG another UC campus (which is highly recommended for all competitive majors like Econ) I would review the TAG requirements to see when your choice campus requires you to finish your Math and English requirements.
For instance, looking at an [url=http://admission.universityofcalifornia.edu/counselors/files/tag-matrix.pdf]outdated TAG Matrix/url shows Irvine, which has a great Quantitative Economics program, has recently required their Math component to be completed the Summer immediately before application!)