1，要回定金我們有沒有道理？
Your right is defined by (1) the initial agreement (2) your state's consumer protection law. You need someone to look at your initial agreement to see under what condition you can get your deposit back, and see if their conduct meets the condition. If the conditoin is too strict, you might seek to void the contract under consumer protection law.
2，如果該要，怎麽操作？隻能打官司嗎？
Small claim court would be approporite.
3，我們認了，定金不要了，他們會不會說是我違約，向我們要違約金？
Again, you need to see your initial agreement to see if the deposit serves as liquidated damage (違約金). If so, you would not be liable for anything else.
The worst case would be, they said "材料已經下了訂單", assuming the material is 'custom made', and cannot be used on other project, you might be liable for the cost of those materials. But again, I think the possibility of them suing you is remote.
