Done properly, mods that keep the original unit/vehicle classnames intact could be used in the campaign without any negative effects.
Problem is as Tyco said, there aren't any mods. The RHS mod is probably the most realistic chance of seeing a satisfying amount of contemporary vehicles and weapons. The downside (if you consider it that) is that the artists over at RHS place an emphasis on quality, so it takes them a while to push anything out. (but it looks
fabulous).
http://forums.bistudio.com/showthread.ph...n-A3-WIP-ThreadOther two prospective options are AiA/CUP and ASM.
AiA & CUP are related initiatives to port over all of the Arma 2/OA/TKoH content to Arma 3 and bring them to the A3 standard. CUP is a related effort intended to improve upon the quality of the ports, and
triesto avoid duplication of work by offering a mediator to coordinate between mod makers (E.G. so we don't have 82.5 M1A2 mods).
Problems with this are that the ports (at least last time I checked it out) were pretty rough hack jobs, the actual unit models haven't been properly redone for the Arma 2 skeleton, and none of the Arma 2 vehicles have proper physix (watching an M1A2 flip over lengthwise after goosing the brakes is always a jarring experience). CUP has the problem that it is relying on the cooridnation of community modders, who tend to be passionate that
their M1A2 mod is the only true mode and are offended that you would even suggest that they do the Lada instead.
http://forums.bistudio.com/showthread.php?174016-All-in-Arma-Standalone-(AiA-SA)-A1-A2-OA-content-in-A3
ASM is an effort by a Polish moder that pretty much tries to do the same thing AiA was doing, but seemed to be futher along in the process.
http://forums.bistudio.com/showthread.php?169068-ASM-Arma-Samples-ModUntil recently it seemed the first release would be ASM, however some vague forum drama apparently arose between the ASM guy and some members of the CUP team. The ASM guy has decided to suspend work on it for the forceable future, because nothing says maturity like grabbing your toys and running home while whining it's the other guy's fault.