When using subnet discovery, I find that I can pull the table from my "routers" but not my layer 3 switches. We summarize our tables so I need to pull subnets from each of my 4507's or 3560's to get the sites "none summarized" routing table. BUT, each time I do this I get zero subnets returned from the layer 3 switches.
you can check if your L3 switches respond to these OIDs:
1 - ISO assigned OIDs
1.3 - ISO Identified Organization
1.3.6 - US Department of Defense
188.8.131.52 - OID assignments from 184.108.40.206 - Internet
220.127.116.11.2 - IETF Management
18.104.22.168.2.1 - SNMP MIB-2
22.214.171.124.2.1.4 - ip
126.96.36.199.188.8.131.52 - ipForwarding
184.108.40.206.220.127.116.11 - ipRouteTable
if so, please send us SNMP walk (use my email) and we will reproduce behavior in our lab.