What did you try so far?
Did you try to verify the integrity of the game cache?

I did the steam config thing. umm. I deleted the appcache. I re-installed. mm. I did everything it said to do.. even took down webroot completely still didn't work. and everytime I verify the integrity of the game cache it says 1 valid something something. then says its going to download something. and everytime it downloads its always 8.6mb after that it'll give me the unknown error sometimes. usually just keeps telling me that I'm missing something.

Hello,
thank you for submitting the ticket. Could you please try to temporarily disable your anti-virus and verify the game data via Steam?
https://support.steampowered.com/kb_article.php?ref=2037-QEUH-3335

It is possible that your game executable is falsely marked as a harm to your computer.

I told y'all that's what I did in the message above yours. Webroot is my anti virus and I took it down to do that. It just does the same thing. Downloads one thing thats 8.6mb and gives me the error still

Could you please try to run Steam as an admin? It seems that something is preventing your game exe to download or there is something deleting it.
Can you try to find arma3.exe in your folder. By default, its location is similar to this one:
C:\Program Files (x86)\Steam\SteamApps\common\Arma 3

I've tried that as well.and I don't see "arma3.exe" in the arma 3 folder

Could you please try to manually delete all the files and perform data verification again? Please watch the folder in Windows Explorer to see if the executable is downloaded or not.

Are you suggesting re-installing the game again? Because I can't. I've already done it twice and I'm on limited data plan. but every time I verify my game data it just downloads the 8.6mb over and over again.

Are you sure the file isn't being deleted after your verify your game data?

Deleted some of the files. trying to verify the game cache. it just keeps downloading 28mb. I think I'm done messing with this bs. Hopefully next month when I can download it again it'll work. buying a game you can't even play really pisses off a person. bsbsbs.

Nothing is even showing up in the folder...

What would delete it? I'm not computer savvy. I have no clue what I'm doing when it comes to fixing this stuff. I'm getting very frustrated.

Sorry for exploding. just this is taking a lot of my time. looking for answers and trying things I've already tried. I did a back up of the game the first time and restored my game. I did now notice something popping up in the folder saying arma3 after I download that 8.6mb and it just disappears. I'm going to try and uninstall webroot completely and try this again "fingers crossed" if that doesn't work then I have no clue what could be deleting the file.

still instantly deletes the file after uninstalling webroot.

So.... I forgot I had mcafee or whatever downloaded... because I never use it.... Deleted that. verified game cache. game launched! :) thanks for the assist.

I really can recommend, that you will get yourself another internet provider. ArmA 3 is a really enjoyable game, if you play it online.

According to your problem:

It seems, that there is a software blocking and deleting the installation of the ArmA3.exe. Its not the developers fault but more a your-system-fault.

Maybe you could attach a screenshot of all installed software (system controls/programs and features). It could be possible to help you, if we know, what programs you have installed on your system.

Arma 3 is now running. No longer need any help.

Glad to see that it works. Could you please provide more details about what version of McAfee antivirus you were using. Deleting signed and verified executables is definitely not correct.