AMD Phenom II X4 965 BE Radeon HD 6870 8GB of RAM 7200 rpm HDD
I just added the things I feel are important. So can that run ArmA 3 on medium-high settings?
The requirements minimum are on CanYouRunit.
From the tidbits of information on the official ARMA forums yes you can. A BIS dev actually at one point commented on those forums that if you run ARMA II now you will run ARMA III one quality setting lower.