Did they have the rights to force me into a contract and how do I know if the contract is legal?

I work for a Computer and network firm. It is a small LLC company owned by a husband and wife. Last week, him and her brought in a contract. It was just one page and it said that for 3 year from the date of unemployment, I could not contact the vendors of the company. The page was pre signed by the wife as the company owner and her husband as witness. The contract seemed harmless and I was told that if I want to keep my job, I had to sign it. So I did. Well after I signed it, she took the contract and made me a copy of it and also handed me a 2nd page and told me that was the rest of the contract and that page included things such as for 5 years from date of unemployment, I cannot work in that line of work with any other company and a lot of extra stuff. I have noticed things in the contract like the wrong address of the office.