I've never done 3x3mi or 3x5k but I once did 2x3mi before I ran my PB 2:32:57. Think I did it in 16:30 and 16:25ish with about 4min rest, and I wasn't destroyed at the end. Probably couldn't have done a third set at the same pace without it being very difficult.
I might try this workout in a few weeks between this weekend's 50k and my goal 50 miler 5 weeks later. I suppose that if I went out and tried it tomorrow, I would start at 5:32-5:35 pace and try to cut down a little bit. I'd do 4min jog between or a half mile. In my mind it doesn't seem insanely difficult, but I guess the first two shouldn't be.