The questions that separate the quotes

"What does your workmanship warranty cover, and for how long?"

Ask for it in writing, in the company name, with a term. Five to ten years is common; the term matters less than whether they will still be trading.

"Can I see a job you finished five years ago?"

Recent work shows tidiness. Older work shows competence. Ask for the older one.

"Who is actually doing the work?"

Employees, or subcontractors? Both are fine, but subcontracted crews change who carries the workers compensation cover and who stands behind the workmanship warranty.

"What happens if the decking is rotten?"

The right answer is a specific price per sheet, written into the contract before work starts. A vague answer here is how a fixed price stops being fixed on day two.

"What underlayment, and how are the valleys done?"

This is the question that reveals whether you are comparing the same roof. Underlayment, ice-and-water placement, drip edge and valley treatment are where quotes quietly diverge.

"What is your licence number, and who holds it?"

Then look it up. You are checking that the licence is current and that the name on it matches the name on the contract you are being asked to sign.

In short

Six questions, three contractors, one afternoon. It is the cheapest insurance available on a job this size.