will a pre-employment contract hold up in court?

This kind of clause can be upheld in a court The main thing the employer would have to to show was that the clause was reasonable.What reasonable means depends on a number of factors relating to the job.You would need to provide an employment lawyer with all of the details and get advice on the reasonableness or otherwise of the clause.One of the factors would be how long you are prevented from working for the competition.Three years sounds too long but without looking at all of the detail it is impossible to say.I wouldn't worry about "what would happen if" unless and till your lawyer tells you it might happen.

Usually they will hold up in court. There are many companies that have contracts like this. The real risk is whether they will spend the time & resources enforcing this contract (requires legal intervention). If you are a relatively low-level employee without key information, I can't see that risk being very high (it could depend on the industry you are in though). I have only ever seen one company go after an employee and he was at the VP level and went to a direct competitor.

Yes, most companies will have something like this to pervent people to carry the knowledge over to competitor. They will held up in court, but it depends on what type of position you are in. If you are just someone who answer phones, then I don't think they will use it to against you. It is very tricky, but mostly it depends on what type of company secret do you know. Let if you know what is the new product for next season etc...