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.