It varies wildly. You should check your touchpad's settings. I've found that in many systems, if it supports simulating a middle-click, that it's disabled by default.
Some let you map a multi-finger tap to middle-click.
Some let you map clicking both the right and left buttons together to middle-click (e.g., Dell Alps).
Some let you define a zone where taps in that area are middle-clicks (e.g., Synaptics).
And there are a frustrating number of touchpads that don't support simulating middle-clicking at all. I hate them all with a fiery passion.
(And sometimes, it depends on the laptop OEM, too. For example, HP laptops that use Synaptics: for some reason, HP disabled Synaptics' tap zones feature, but with some registry tweaking, I was able to hack tap zones back and get my middle-click.)