It is probably not illegal for your wife's employer to terminate her if she is unable to work for three months. The Family and Medical Leave Act is sometimes used for maternity leave, (and may allow up to 12 weeks of unpaid leave) but it sounds like this employer is much too small to fall under the jurisdiction of such law.
Unfortunately F.M.L.A., is usually what is utilized for maternity leave. In order for F.M.L.A. to kick in a company has to have a certain amount of employees. It appears your company is very small.