Income works on several things.
- Win or loss streaks.
- Interest (likely where he pulled way ahead of you)
- Wins.
Basically, the reason you have all these three stars is because you likely blew all your income early to 3* champs before opponents could roll through them. You played the early game and did well enough with it to get rank 3.
Others gambled for late game saving for interest, and possibly even using losing streaks to counterbalance any winning streaks you had AND get pick priority. It's even possible they got 2 gold spatulas which means another champ slot.
In short, you played for early game, they risked everything for late game and managed to pull it off, which doesn't always work out.
Also, that mode is kinda RNG trash, coming from someone that's played a lot of ARAM.