How important is the build number on a SQL Cluster
You probably don't need anyone to tell you that you need to fix the build level on that third node! I suspect it's all working ok because of the introduction of the resource database in SQL2005:
http://msdn.microsoft.com/en-us/library/ms190940.aspx

I vaguely remember an old Windows 2000 / SQL Server 2000 cluster I used to work with, where SQL thought it was a lower patched level on one node compared to the other.