MDT DESCENDED TO WRONG ALT AND HAD LESS THAN STANDARD SEPARATION WITH AN LTT DESCENDING TO SAME ALT.

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WHILE ON VECTOR HDG AND AT ASSIGNED ALT OF 7000' SGF APCH CTL INSTRUCTED AS I AND MY F/O RECALL; 'DSND TO 3000';' WHICH WE DID. AT APPROX 4 MI W OF THE FIELD APCH ADVISED US OF AN LTT Y APCHING FROM OUR 10 OR 11 O'CLOCK POS DSNDING OUT OF 2800'. AS WE HAD VISUAL CONTACT WITH THAT ACFT THE APCH CTLR TOLD US HE HAD DSNDED US TO 6000'. WE TOLD HIM THAT WE HAD VISUAL CONTACT WITH THE LTT Y AND THAT WE HAD UNDERSTOOD OUR DSCNT INSTRUCTIONS TO BE 3000' AND THAT WE HAD READ BACK 3000'. WE TURNED LEFT TO AVOID THE LTT Y FLT PATH AND THEN PROCEEDED WITH A VISUAL APCH TO THE ARPT AND LANDED AS CLRED W/O FURTHER INCIDENT. I IMMEDIATELY CALLED THE ATC CTL TO DISCUSS THE INCIDENT. THE SUPVR SAID THAT THE TAPE HAD NOT BEEN REVIEWED AND THAT THE CTLR WAS BUSY AND COULDN'T COME TO THE PHONE; BUT IT WAS POSSIBLE THAT AN ERROR HAD BEEN COMMITTED AND SAID THAT WE PROBABLY MISUNDERSTOOD OUR INSTRUCTIONS. I TOLD HIM TO REVIEW THE TAPE. HE SAID HE WOULD AS SOON AS POSSIBLE. THE CONVERSATION WAS POLITE AND CORDIAL. SGF ATC HAS ALWAYS BEEN EXCELLENT AND I FEEL THAT AN INDIRECT CAUSE WAS HEAVY TFC WORKLOAD AT THE TIME.