1. Both 2. Either. Just make sure it's the "Ubuntu" version and matches your OS 3. AVG and Sophos support GNU/Linux as well I think
If you want to stay "fully-free" (and you should, because you're using Trisquel!) you're better off with ClamTk. You can install it through the "Add/Remove Software Applications" panel in the Start menu.
I have some more thoughts on whether you need GNU/Linux anti-virus here: https://bobstechsite.com/make-gnu-linux-desktop-secure/. But the tl;dr version is locking down your browser is far more effective on a GNU/Linux machine. (eg. install an ad-blocker, disable 3rd party cookies and disable JavaScript by default)
For home use you'd install AV on a GNU/Linux machine to prevent it becoming a "safe harbour" for Windows viruses. If you have a GNU/Linux-only network, you probably don't need AV.
