"How good is free system security software?"
It's perfectly good - and the 3 programs you mention are regularly supported/recommended/praised in these Forums.
An alternative (equally good) free AV program - which I use - is Avast.
You might also want to try Windows Defender anti-spyware from Microsoft.
There is absolutely no reason at all for the home user (or small network user) to waste money on AV and anti-malware programs.