You said you already have the "Platinum card", but is that the AmEx Platinum or Delta Platinum AmEx? Signing up for the Delta Platinum AmEx will get you 5,000 MQMs (and 35k RDMs) after $1k of spend, for a $195 fee. Unless there is a signup offer I'm not aware of, the Delta Reserve AmEx will get you 15k MQMs, but after $30K of spend, not $3k.
Edit: Oops, you need 6,000 MQMs, I read it at as 5,000.