I’ve done Relentless twice, once solo (wrists failed at 15hrs) and once as a pair.
3 weeks to go, i personally recommend very little in the way of longer rides, base should be well established by this stage of the season, and there’s little you can achieve other than fatigue in 3 weeks.
My advice would be to spend 3 weeks fine tuning. It’s not a course where the traditional advice of pacing yourself and avoiding efforts above threshold works. Last two years its been a really hard course with multiple steep sections that force you into the red. Work on making efforts and recovering from them, and knowing what foods you can tolerate when being forced to work harder than you’d ideally like to.