The answer to this question depends on how these taxes are raised.
Currently, this tax is raised in the form of fees, due quarterly (or per semester) for the four (or more, or less) years of UC education consumed. Paid for by students and/or their families. Payment plans, subsidies, and loans may be available to pay this tax...
(I know I'm being a bit colorful, but I hope you catch my drift)
Those don't sound like "taxes" in the sense that they're paid by the entire population, they're just direct payments to the school and only paid by students and parents (those who directly benefit from the service of education).
What I am trying to say is that, from an individual perspective, it makes more sense to pay the $15k. If you're making $50k/year in income, 3% of that for the rest of your life (assuming you work for about 50 years) comes to about $60k, which is what 4x$15k is anyway. However, if your income is above that, you pay significantly more. The US median income in 2014 was $51939 (got this number from a quick Google search, might not be 100% accurate), so half the population would indeed pay a total sum larger than $60k with a 3% tax.
Currently, this tax is raised in the form of fees, due quarterly (or per semester) for the four (or more, or less) years of UC education consumed. Paid for by students and/or their families. Payment plans, subsidies, and loans may be available to pay this tax...
(I know I'm being a bit colorful, but I hope you catch my drift)