Project was awarded but for unforeseen reasons has been delayed approx. 3 years.
Thus, original bids from subcontractors are 3 yrs. Old. Customer has requested updated "expected" cost for project.
Need to write subd a letter asking for their "new" updated project cost with detailed justification.
Any suggestion as to format?

They are most likely looking for taking advantage of the economy downturn during those 3 years. Otherwise they would have asked you to honor your original bids. Just ask for a new bid from them as if the old bid never happened, as in what is your price today for the work described 3 years ago. I would think that they want justification mainly if the price is higher and it could be based on present workload, material costs etc. You know the owner's situation and the bid specifications- I don't. Does the owner have the option to just put the project out for bids again? Maybe you should talk to a lawyer familiar with contract law first. It is almost like justification for a claim on a construction project.