I now have three computers, can I use my new software on all legally?

All the computers have different internet connections under three different accounts.

I now have three computers, can I use my new software on all legally?virus

It all depends on the license, open source software you won't have any problem. Some licenses only allow the installation on one machine only, and some allow you to install on as many machines as you like but only use the software on one computer at a time, you would need to read the terms %26 conditions carefully (if you want to stay strictly to the rules).

If you are talking about windows then I think it's one license per machine.

Also I don't think it is illegal to break the license agreement on any piece of software, so the police won't cuff you %26 take you in, but you would be open to civil action where the software producer could claim compensation.

I now have three computers, can I use my new software on all legally?anti virus

You are supposed to have a copy of software for each technically.

Unless you are using them for some kind of business though, most people don't really care, I load my software on as many machines as I feel like (including some of my friends).
Whenever you install a software, the initial display that pops-out at the beginning of the installation is the EULA (End-User License Agreement) which is a long text most of us never bother to read. But it actually defines the detailed terms and conditions under which you can use the said software legally. At the bottom of the display it asks you whether you "Agree" or "Disagree" with the terms by clicking on the relevant button.