15x400 @ 3k pace is a hard workout.
No point doing 6k total @3k pace unless your are at sub elite level in my opinion.
Wrong. 15x400 can be done at 3k pace with 100m to 200m jog of recovery.
It's way too hard with 100m recovery jog (25-30s).
200m (50-60s) jog would make it a difficult but doable workout, but there are better 3k workouts with the same level of difficulty - 6x800 or 4x1000+a couple 400 @ 1500 pace with 3' rest would probably prepare you better for a race.
You are both right and wrong.
Yes, 15x400 can be done at 3k pace with 100m jog (40-45s). The key is to not do a steady jog at 25-30s as you suggested but really slow shuffling, just enough to keep HR elevated.
But, not every runner can do it. It depends on FT vs ST runner, VO2MAX vs lactate threshold/400m speed, lactate clearance rate etc. Brynn Brown would have absolutely no problem with that workout (she even did 4xMile at 3k pace on only 2:30 rest).
The most important thing to know is that between 3k and mile pace there is a huge difference, especially for ST runners with high VO2MAX but low speed. They can do a lot of work/volume at 3k-5k pace, but anything mile-pace or harder kills them with low rests or in moderate-high volume.
I mean ... a 45s/100m jog is 12 min mile pace, which is basically a walk ... Normally a jog rest in a workout is around 8 min mile pace.
Also, that workout you're referring to was at 5k pace, not 3k pace. She ran a 9:39 2mi, which is equal to a 15:30 5k. 4xmile with 2:30 r is a doable workout. 4xmile @ 3k, not so much.