Becoz I guess for the result ..head to head.. even though You won 1 and lost 1 to them but the opponents scored more than you when you three faced off so that's why U ranked 3 even your goal differences better than them...
For instance..
My 3rd team FC Sir Dang and another team finish same 15 points..I won 1 leg n he won the other when we faced off..
But at his home, I won 4-1..n back home (I did not watch the match.. and I lost 1-2.. I had 2 players who received red card n 1 injured)..
so the goal differences between us is 5-3 for me..so even my total goal differences only 24 and he got 27 after completed the group matches (we both won other teams)..I still finished top of the group...ahead of him...
Hope it helps...and have a nice day!