First time chicken owner: can I put them outside?

So, let me start off first by saying that I had a tub too for the first couple of weeks and they outgrew it and I had to build an extension, so be prepared for that Ok, so I personally would wait until they are at least 7-8 weeks old, being that there are only three. Never put a heat lamp in the coop outdoors, it's too risky. When are you getting the chicks? because if you are getting them in a couple weeks by the time they go outdoors it'll be warmer.

I had 8 chicks and live in Northern CA(so around the same temps. as you and put them outside when they were 10 weeks old).

 

I suggest you read Blooie's thread just above about heating pad brooding. It's the perfect solution to your concerns and perfect for brooding very small numbers of chicks.

This past year, I brooded two batches of chicks outdoors in my covered run under the heating pad system. The first batch was four and the second was three chicks. The temps for the first group was very similar to what you are experiencing there in N. Cal. The heating pad is much safer than a heat lamp in every respect, and it is the perfect and natural heat source for chicks that closely resembles a broody hen.

Keep in mind that broody hens raise their chicks sometimes in below freezing conditions, and the chicks do just fine. My chicks had temps in the 30s at night and not above 50 in the daytime. The did splendidly, feathered out quickly, and were very cold hardy at a very young age.

The best part of raising chicks in a coop is they are already home, and don't have to go through the stress of moving from a confining brooder to a large coop. Chicks hate change.

The other big plus is you don't have to put up with all the fine dust that chicks generate. I had dust so deep in the closets of the room my chicks were in that it amazed me how it got there, but the dust and dander gets in everything. Trust me. You want to grab the opportunity you have been blessed with and avoid that mess! And your chicks will benefit in ways you can't even begin to imagine.