This is the most effective build that I have ever used for Pantheon in 3v3. Its espicially useful against melee Champions becasue of the Attack Speed procing the BLOCK passive. To make this build really work, you must play aggressive in lane and dont be afraid to purchase a couple of HEALTH and MANA potions every now and then. If the enemy team is heavy crowd control capable, you might want to purchase Mercury Treads instead of Berzerker Greaves for the tenacity. Because with the exception of FLASH, you dont have a really good escape mechanic.
The skill sequence is SPEAR SHOT > AEGIS OF ZEONIA > HEARTSEEKER STRIKE > SPEAR SHOT . Rinse and Repeat this, but be smart about it. Your attack speed/passive will help mitigate the damage done to you. Use your ULT to trap enemies between you and your teammates if you can. It puts them in what I like to call "Oh ****! mode". Nothing screams "I'm about to die" like being trapped betwen an angry Spartan who fell from the sky and a psyco clown or crazed mage.
If you play this build right, enemies will fear you and even think twice about taking you on 2v1. You will get **** about "Panth is OP"!!!! You must BE AGGRESSIVE and make them refer to you as the boogie man!