The highest level Allosaurus will become an Alpha if at least one other is nearby; you do not need 3 to gain the Alpha Bonus.
The best way I find to breed Allo's is to find a pack with 1 150, kill off the other two and tame a 150 Allo. Do this for 2 150 Allosauruses and you can breed a nasty army of these things. You'll be able to take on Rex's like they're dodo's. Wyverns make for east kills, too. Just watch out for Alpha's.
The only thing a good pack of Allo's cannot take on, is a wild Giga.
This dino is by far my favorite. If you can manage to find 3 120 allos you will be close to unstoppable. When you have 3 allos( doesn't matter what genders) the highest level will be considered the "alpha", this means the alpha can do bleed affects which causes damage over time as well as slows you down temporarily. When getting a pack of allos or a allo the only stats that ever need to be pumped is stamina, melee damage, and movement speed. Hope this helps! 😄