I have a 2000 DTS, it shifts find through 1st, and second, but I notice that third takes awhile, I do about 60-65 and I'm doing like 2800-3000 rpms, is this normal?

Check for codes.

No codes,only code it for the catalytic converter


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No codes? keep checking.

Sounds like your high gear may not be engaging.

what is the mileage, maint record, etc...

Restricted cat conv can cause transmission concerns, not always setting codes due to TCC not enabled. Because of engine data not allowing it. FYI ECM & CAT.CONV. carry 8yr / 80k mile federal emission warranty.