when I was still a middle-distance track coach (early 2000s) I got to hear a presentation on “The Mystery Zone” by a USTAF coach. I was using an Accusport with my guys, and the presentation was on the huge variation in lactate response for all-out efforts from 60 sec up to 4min.
Some slow twichers may be pretty aerobic for 3min. some may be as much as 70% anaerobic for 3min (think pursuiters who are also good road sprinters, or Johnny Gray-type 800m guys in track).
Because of that huge variation, you can’t come up with a good “rule of thumb” number for what 3min power is as a percentage of FTP. All you can do is look at one particular athlete’s PD curve and see what it is for them.