FWIW, 3x3's are designed to build the aerobic engine in the 2nd set. You're depleting in the first set to force your body to utilize a different energy source for the 2nd half. This is why the first few reps always feel fine and the last few damn near impossible. Enabling you to do higher power threshold intervals later since your body will have built up some more capacity.
Anaerobic or real VO2 would have more solid rest interval and maybe even only up to 2min per rep.
I do nearly all my "interval" based training indoors on a trainer. Anything from 1x60min SS down to 1min on 1min off sets. Until the power surge or demand on the trainer would make it useless to do indoors. So sprints and under a minute I go outdoors. I just really don't like the idea of an outdoors influence screwing up a 3x8 or 3x3 set due to having to soft pedal for a second for some BS car or roller or something.
FWIW, a beginner shouldn't try to formulate their own plan. Just adopt an existing plan to your lifestyle or find one that fits your needs. There's a lot that goes into it. Or get a coach.
Also, you say sportive. It's fine to do some hiit to save time when life gets in the way, but that's a rough cornerstone use if sportive is your thing. I'd get in hours and miles and maybe once in a while do something saucy or more difficult. Nobody doing a sportive "needs" to do intervals that might make you want to puke. You need to be able to so some sweetspot intervals in the middle of a longer endurance ride, that'd be useful for sportive. A sportive rider isn't likely going to be able to climb at threshold multiple times like a pro can.