If this includes an addition, then I'd start with an architect. You can always get additional advice about the plans for the interior space, especially if no load-bearing walls are involved. You'll almost certainly want all three before the project is done, though, so I'd start collecting references.

We did two renovations and in both cases started with an architect. He was the best person to talk to about how we wanted to use the space, the needs of our family, our entertaining habits, etc. Money well spent. After that we used a GC who works a lot with the architect. He picked up one or two things that the architect didn't cover. It was all pretty seamless.

With a gut of the 2nd floor and an addition, you're looking to undertake a pretty significant remodel. If you are thinking of changing any of the rooms or moving walls around, an Architect can help you get the most out of your space and help you see potential ideas that you may not have considered. They also help you to think more in terms of what you hope to do with the space instead of what constraints might be there (location of electrical, plumbing, etc) that a builder would see first. You might find this link which talks about the roles and responsibilities of each during the remodel process helpful as well.

Your other option would be to use a design/build firm that does both the upfront design work (not interior design - would be space design and blueprints) and also does the build. These firms are more likely to know the costs to build and can help you stay within a budget during the planning stage. We see lots of plans every year that cost out 2x - 5x more than the homeowner's budget. Not that the plans weren't beautiful, the people just can't afford them and then get angry at everyone in the process because they paid so much for the plans. There are some lucky people for whom budget is not an issue, but it is rare.