It’s pretty simple. UCB and UCLA typically accept 10-20% of students of any CA HS, yes competitive or not (so it’s actually better to be in a non-comp school, others being the same). UCSD accepts up to 30’ish%, including those also accepted to UCB and UCLA. Now Eng/CS majors are generally a lot more competitive than L&S majors so it’s likely that even if you are in the top 5% of your HS you still can’t get into UCLA/UCSD CSE (or UCB EECS for sure).