I am moving out at the end of my lease and my landlord sent me a letter stating that I have to have the carpets professionally cleaned. I've lived there for 3 years and was planning to clean the carpets myself. Can they force me to pay a professional company?

If you don't, they will have their firm come in and deduct it from your security deposit. Better if you control the cost.

Reread your lease. If this requirement is not in the lease you can do it however you want. Carpet cleaning is not normally a requirement at all if it is just normal wear and tear.

While I don't disagree with ballenger, they will still have their own firm come in and clean and deduct it from your deposit. Their firm will state that the carpet needed it.