Green or Gold?

[Chris Moore]

What’s to stop me from stitching the two 1.5tb together and mirror against the 3tb?

You could only do that by creating the zpool from the command line. You would need to create a partition on each of your 1.5TB drives of a size such that two partitions of that same size would fit on the 3TB drive. Then you create the two partitions on the 3TB drive and mirror one of those partitions with the partition on the first 1.5TB drive and mirror the second partition on the 3TB drive with the partition on the second 1.5TB drive. This would protect you from the failure of either of the two 1.5TB drives or the failure of the 3TB drive.
This is not a recommended solution. It would likely have poor performance. I will not tell you the details of how to do it, and if you do it, most people will not try to help you fix it when it breaks. Everything breaks eventually.
Just because you can do something, it doesn't mean that you should.

 

[Chris Moore]

I like 2 drive mirrors. They are simple and, in my opinion, should be safer than striping multiple drives. Of course theory and practise are two different things. I have no experience with raid 5s and 10s, etc. I have read the design of striping multiples complicates things.
K.I.S.S.

It sounds like you need to read the "Intro to ZFS" to get a better understanding of the technology:
http://doc.freenas.org/11/zfsprimer.html
There are a lot of links, you probably need to explore those too.