I have worked at my company for over ten years.Only now am I being asked to fill in timesheets.Nobody else has been asked to do this.Admittedly there are only three people at the company at present,one being my boss.Still, I would like to know if this could be considered discriminatory by my employer.

Is it discriminatory to have to fill in timesheets if I am the only one?
Answers
Items to think about: Are you salaried or waged? Did State or Local laws change?
You really should address this to an Attorney licensed to practice and specializing in Labor/Employment Law in the area in which you live (laws differ).
Anon
What type of time recording are you doing?
For example, does your boss want to know how many hours you spend on sculpting or modeling each job, so s/he can bill accordingly?
Are you recording non modeling time, eg meetings, time off, training/education.
What does the 3rd person in your company do?
You may consider time reporting a nuisance or pain but it is not discrimination. Lee's example about job costing and, by implication, product pricing, may be a more likely reason.
What troubles me about your query is that it indicates a lack of communication. In a company with three people, communication shouldn't normally be an obstacle. But, like many things in life, effective communication takes effort from everyone involved. What explanations were provided when you were asked to start doing timesheets? Did you ask for any?
Your boss doesn't "owe" you an explanation but it would have been good management practice to provide one. That helps you to be engaged in managing your work to help the company benefit from your knowledge and skills.
Here's what I would do next, instead of worrying about discrimination. Ask for specific reasons this policy was adopted. Frame that request in the context of trying to understand the goals of the policy. If you know the objectives, you should be better able to help meet them. There may be times you can improve the breadth or quality of your communication or reporting, beyond just the providing a number, so that your response is even more meaningful/useful.
If there are no rational, considered answers to your query, then it may be time to ask whether there are concerns about your job performance, whether the company is having financial issues, etc. Listen carefully when you ask these questions. IF there are perceived performance issues, identify them and mutually develop a concrete action plan, with timelines and measurable results, to address them.
I agree that you should ask for an explanation of the policy. The most likely explanation is that you are a "nonexempt" employee covered by overtime rules, and your boss is "exempt" and not entitled to overtime pay. This is a complex area of labor law, and the source of plenty of lawsuits (usually from mid-level employees who want to be classified as nonexempt and therefore entitled to be paid for overtime) so, as others have suggested, if you don't get a satisfactory answer to your concerns, you would probably need to find an attorney who specializes in this area.