Get the advice of your architect.

Works with all 3 won’t really differentiate.

You're gut may be the best decider. I am sure one stands out a bit more there. If they are really all equally respected and vetted one typically should just feel right over the others.

millworkman is correct--if all three are the same in terms of competency, ability to work with your architect, and in the same general price range, it comes down to chemistry. With whom would you rather work? Which builder seems most eager to undertake your project? How responsive is each one to your questions and concerns? Make sure you know who the project manager is, meaning the person you will deal with on a day-to-day basis, and evaluate how well you can deal with him/her. Building a home is a very exciting process--good luck with your project!

The short answer:

You've indicated your intent to choose between three equivalent custom builders based on cost, where each builds on a cost-plus basis. That's easy: just ask them what the "plus" part of cost-plus is and you have your answer. Alternatively, you could ask them to execute your project on a fixed management fee basis. Low fee wins. Or flip a coin, because your architect can work with any of them.

The longer answer:

You're not buying a product. You're hiring a service provider whose service, in turn, produces a product. Consider how you select other service providers; your dentist, doctor, attorney, accountant, mechanic, home painter, barber, landscaper, your... well, you get the point. Even if all three builders bought their materials from the same sources and used the same trade contractors, it's not likely they would all provide the same level of service and quality. Do you want daily updates? Weekly on-site meetings? Someone on site at least x hours/day supervising and coordinating the work or is a daily drive-by enough? Do you want really neat, clean and safe job site? Do you need a builder who is available and willing to answer your questions daily? Does the construction need to be truly managed to achieve particular objectives for cost, quality, and schedule? Weekly invoices? Or is okay to take six months longer than one of the other builders? Perhaps you don't need to or can't be involved in the day-to-day stuff and won't be on site until move-in day.

I think you'll be best served by determining what's important to you, evaluating each builder's processes vis a vis those needs and determining which best fits your particular needs.

Best wishes for a successful project.

The #1 complaint in construction is poor communication. From small to large, that’s across the board. The expert doing the job doesn’t communicate the process, the costs, or the consequences, to the person who hired them.

It takes a lot of experience to develop processes that DO communicate, and yet don’t slow down the pro significantly, or make them feel like they are unnecessarily teaching kindergarten building. Or that doesn’t make the owner feel like they are being condescended to.

Bri talks about ”information asymmetry” and thats the root of the typical communication issue. The expert slways knows more. Its why you hire them. But you need to know enough to make good decisions. How does that occur?

It’s a fine line to walk, and no matter the effort on both parts, some friction will occur. How is the friction handled? Resentments or grudges never help. You have aligned goals. How do you reach those goals, and smooth over disagreements? Is there a process that is created for disputes, as well as a clear communication path to help prevent them?

Talk to current clients about your own important issues, but ask about communnication process. Because that is what most things go back to. The best builder in the world, who doesnt tell you what he is doing, even if he is doing everything right, makes you feel like an uninvolved roadblock rather than an integral part of your own build.

Call all the references, then decide.

Also make sure they've been in business for more than a while. At least 10 years.

I'd also check references regarding lumber yards, etc. Make sure to see if any are in arrears, etc.