This really isn't that hard to understand. The link posted with the question's answer is plainly obvious.
In the Workgroup column on the row for RAM, the limit is clearly stated at 3GB.
This has nothing to do with the OS installed on or the amount of installed memory in the system.
You are absolutely right, the link you provided clearly states the 3GB limit! However, as many of the others already complained about, the BOL page for SQL Server 2005 clearly states for :
"SQL Server 2005 Workgroup Edition
Pentium III-compatible processor or higher
Minimum: 600 MHz
Recommended: 1 GHz or higher
Minimum: 512 MB
Recommended: 1 GB or more
Maximum: Operating system maximum"
Clearly, a conflict in which documentation link to believe!
"Any fool can write code that a computer can understand. Good programmers write code that humans can understand." -- Martin Fowler