Development on ATP3 was discontinued after ATP: Dimensions was born.
Which didn't come to full completion.
The closest thing as a replacement is FoundationTechnologies, by Dasher and myself.
It's a plugin driven Technologies Platform, containing most of the popular ATP(3) technologies, such as Phase Cloaking, Ablative Armour, Reflection/Corbonite shielding, and various projectile technologies (which can be customized on a per projectile basis).