According to this information it’s true, but not for all Creative Suite owners. It appears that if you bought Creative Suite 4 Master Collection or Web Premium after September 15th 2009 you can snaffle yourself a free license for Flex Builder 3.
Just follow the instructions (reproduced below, but you’ll need to go to the site to proceed) and Adobe will send you a serial for Flex Builder 3 – pretty cool huh?
Adobe® Flex™ Builder 3 Standard serial number request
As a Creative Suite 4 Web Premium or Master Collection customer, you are eligible to use Adobe Flex Builder 3 at no additional cost. You will need to first download a trial version of Flex Builder 3 and then activate Standard edition using the serial number we email to you.
Proof of Eligibility
To receive a serial number to unlock the product, you must prove that you are a Creative Suite 4 license holder by uploading an electronic copy of your Creative Suite 4 Sales Order Receipt from Adobe or from an authorized Adobe reseller, in addition to providing a valid Creative Suite 4 Web Premium or Master Collection serial number. A Flex Builder 3 Standard serial number will then be emailed to you.
Terms and conditions:
Flex Builder 3 serial numbers are only available to customers who have purchased a copy of Adobe Creative Suite 4 Web Premium or Adobe Creative Suite 4 Master Collection on or after September 15, 2009. By submitting this request, you are confirming that the serial number you enter has been obtained by acquiring a valid Creative Suite 4 license from Adobe or from an authorized Adobe reseller. Once installed, the use of Flex Builder 3 is restricted to the license holder of Adobe Creative Suite 4.
So just head over to https://freeriatools.adobe.com/cs/ and start the process rolling :D