I presume the existing tunnel is between the 3002 and the 3005, not the PIX, correct? What version are you running on the 3005?
There is a bug in 3.6 code where the concentrator will just blindly strip off IPSec headers on a packet until it gets to a non-IPSec header, even though those headers may not be destined for it specifically. For example, it would strip off the IPsec header for the 3002-3005 tunnel, but then it would see another IPSec header for the PC-PIX tunnel, rathern than forward this on it will strip off this IPSec header also, the ndrop the packet cause it's not valid for it.
Bug ID is CSCdz38146, details are here:
http://www.cisco.com/cgi-bin/Support/Bugtool/onebug.pl?bugid=CSCdz38146&Submit=Search
Upgrade to one of the Fixed-In Versions and it should resolve your problem.