There will be 3 separate discs in 1 case.

Total game size? According to the store page, Black Ops III requires 60 Gb of disk space.

Minecraft itself is safe  so long as you download it legally from Mojang's official website. But you should be cautious while downloading independent mods and data packs.

You can download Minecraft Java edition for free on Minecraft Official Site . After signing in or creating an account, you can get the game for free. Note that to play it, you need an account with Minecraft purchased. You can get a free trial on the Android and Windows 10 versions of Minecraft Bedrock Edition.