You do not "purchase" the latest versions... they are monthly rental only via the Cloud (stop paying, the programs stop working)
You may purchase any or all CS6 programs at
you've been very helpful
Strictly speaking, there is no cheaper.
It's a matter of how you prefer to pay and whether you want the Cloud (monthly fee) or you're happy with superseded CS6 versions (pay once upfront and own).
The Cloud is available via subscription only. As a CS3 owner, you qualify for the discounted rate of US$29.99/month for the first 12 months. After that it's $49.99/month.
Or you can buy full versions of CS6 software and own them. Your CS3 versions are too old to qualify for upgrade pricing.