You own both machines so ultimately only you can know which fits you better.
If you're perfectly happy with the performance of the Surface 3, then sell the Surface Pro 3.
I have a Surface 3 alongside my Surface Pro 2. My Surface Pro 2 is my favorite machine of all my machines. I use my desktop for video rendering, recording, and hardcore gaming. I use my SP2 for pretty much everything else. The SP2 feels so fast that it's like a desktop replacement. I never feel like it's inferior to my desktop except when I play games.
My Surface 3 is more my luxury machine when I want to read a book or edit a document on a lark. I'm kind of a power user. So to depend on my Surface 3 as my primary machine, I wouldn't be happy. It's not fast enough as a primary machine for me. I also still prefer the pen on the SP2 (love the ability to flip the pen to erase rather than holding a button). To me, a primary machine should be a machine that you can throw any kind of workload at it and it can handle it well.
Don't get me wrong. I really like my S3. But for me, it can't be my primary machine.