SNMPv3 is different from the previous versions. In version 3 you no longer use community strings. Instead you specify a username with authentication password and possibly an encryption key. (depending on whether you just want to authenticate the SNMP traffic or also encrypt it).
So first on the Ironport appliance enable SNMP and configure the SNMPv3 user. Then go to SolarWinds and add your Ironport device with the SNMP user credentials you created.