Since you are in a warm climate, you may still get good trip mileage. It depends upon if your car wants to do autostop soon on your trips. I find that in the cold months (Bay Area) even with warm air mods it takes me about a mile of idling (my first mile is downhill) or longer to get the engine heated up to do autostop. In the summer, I get autostop almost right away and fuel cut right away.
In the service manual it says you don't get autostop for some fixed period when you first start, but it doesn't seem to apply for me in the real world driving. It seems just to want engine temp to hit about 170, and then it will go.
I say autostop, but it also won't be doing lean burn if the engine isn't to the min temp (around 170).
I am sure you must drive other places as well, so you will put more miles on that just that one 3 mile trip.
What's your annual mileage you put on your other car(s)? That's maybe a better figure to look at than the commute.
If you lived in a really cold place, and all your driving were 3 mile trips, then the Insight wouldn't do much good. For you in warm SoCal, even short trips won't be so bad, I think.