Collect AC Emblems
To grab an enemy's Emblem once you've defeated him, press START + SELECT at the victory screen, and their Emblem will be added to your list.
Pressing certain combinations of buttons during gameplay will allow you to drop parts. Hold all shoulder buttons and TRIANGLE to drop your right arm weapon, back units and inside parts. Press all shoulder buttons and click in on the left analog stick (L3) to drop Extension parts. Also, hold all shoulder buttons and press CIRCLE to drop left arm weapons.
In AC3, you can no longer obtain the plus abilities by going into debt. Instead, you must beat the game first. Load your cleared save and check your mail. You should have a message about an optional part called "OP-Intensify" in the shop. Buy it and then kill the guy mentioned in the message. Doing so will unlock one of the PLUS abilities.
The other PLUS abilities are hidden throughout the missions and arena. Beating a certain mission a certain way, or defeating an arena member will activate new PLUS options. These abilities will only work so long as you have the OP-Intensify part installed on your Core. The only drawback to using OP-Intensify part is that it uses all of your optional parts slots.