I've always gotten near 5% response rate on average. Make sure you know your target market, develop a quality list based on more than just need. Many list providers can also filter your list based on income. When you identify a client in need, but also has the means to afford it, you stand a far better chance of generating interest. Once you understand your target market and have a list, make sure your message is clear, and speaks to the needs of your target audience. Incentivizing them with a reason to act soon will also increase response.
Make sure the brochure is appealing to the eye and professional. Since this is your way of introducing them to your company, it's good to put your best foot forward.
Also, it take a few times for them to notice your advertisement in a pile of mail. eBigman had it right, take that 3500, trim it down to 1000, then send 3 variations of your brochure to that thousand people over the course of 4-6 weeks. That will also increase your response rate.