Currently, the answer is 'none of them'.
We have had a few requests pertaining to Level 3 data, and we would be more than happy to support it.
Sadly, Level 3 data seems to vary depending on the gateway/processor combination you choose - and none of the gateways have detailed documentation on which fields are required to qualify for Level 3.
This fact has made it difficult for us to proceed with implementation, but if anyone were to provide the specific fields that we'd need to send to qualify for Level 3 at their processor, we would certainly be willing to look into it.
So if you would like a certain gateway or processor to support Level 3 data, and you have specific information on which additional fields are required to do so, please contact us at firstname.lastname@example.org.