On Titan Loot tier, you will get 3 ascension item for Tier 9 or above.
So if you want to get 3 ascension items, here is the rank you will need to get based on different Titan level, and how many member in the alliance will get 3 items
6* - A+, only 1 person in alliance
7* - A, 5 members in the alliance
8* - B, about 10-15 members in the alliance
9* - C (player must do at least 1.0% of the Titan’s HP in damage), about 90%-100% of the member
10* or above - D, Anyone who done any attack, 100%.
So if you want most of your alliance to get 3 ascension items. You need to hit 9* or above. On the other hand, if you going to fight 10,11 star titan, you really need a team of fully leveled 5* heroes or need to use lots item in order to stay alive. Those titan can kill most of the 4* heroes in 1 hit.
Now for 9* heroes, your 5* heroes can last for 3-4 hit and 4* heroes can take 2 hits. with the help of low level items (potions, axe or turtle banner) your team can last a full 90sec with the titan. So for me (F2P player), 9* titan is kind of the sweet spot for the whole alliance to get 3 ascension item while keeping item usage relatively low.