(This is my edited version of WG Botter 1.0 by Nesox, all credits for making the plugin go to him, I just modified it a little.)
Removed the LeaveParty() command from the Lua.DoString() function call in WGBotter.cs so you can stay in Wintergrasp Party to have a change to get rank 3 - and thus three Marks of Wintergrasp.
People will always leave or (fake)d/c after the Battle for Wintergrasp, so it shouldn't be suspicious with you staying in the group.
Source is included, compile it in Microsoft Visual C# 2008 Express (which is free), if you don't trust me.
Download link: WG-Botter-1.0.1.rar