Some info about scopes.
You have three different type of scopes that are interesting for you:
Mil-dot
PSO type scope
Bullet Drop Compensation
I am not a expert on this, so those might not be the official and correct names of those systems, but you will know what I mean.
Mil-DotThis is more of a system for sniper optics and I dont recall seeing a scope that is using mil-dot optic with a combination of a red-dot or something like that. However I think it is not bad to know how these actually work.
Looks pretty simple and if you have no idea what it is about, it will probably leave you confused.

To make them work you have to do a bit of math, but it is nothing complicated and thankfully every person in ArmA 3 is 1.80m tall. So what you do is cou get on target and see how much mills the target covers. Then you do a bit of math and you get estimate the distance to the target and range your scope accordingly.

You can shorten this calculation by just doing 1800 x covered mills.
PSO type scopes
My personal favourite because you baretly need to zero your scope and finding the range to the target is a bit
easier. Here you have the scope.

In the middle you have bullet drop compensator marks. The only difference to the one you can find a ACOG or the LDS is that it doesnt show the meter marks. At the bottom of it you have a rangefinder that works similar like the mil-dots. The main difference is that it will do the math for you.

You get your target in sight and make it fit into the rangefinding scale. The number show ranges from 200 - 1000m and so you simply have to check the number and you have the range to your target. Combined with the bullet drop compensation marks you dont really have to zero your scope most of the times.
Bullet Drop CompensationThose dont allow proper rangefinding inside the scope, so you need to rely on your skills to estimate distance or you haveto guide yourself in. Pretty much all of those scopes on the game (as far as I know) are zero'ed in at 400m. Like in the other scopes the numbers each represent 100m. So the 6 means 600m etc. If you cant really estimate the distance to the target you can at least shoot it and adjust your fire on where the bullet actually landed.