(almost) Never One - it requires too much brain work to figure out what's going on when scanning through code.

Never three - all that is seen is the condition; the action gets lost.

Most often Two. The action is obvious and generally the most important thing. If the condition is really important I'd:

if ($var) {
   sub ();
}
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to make the condition obvious and the action easy to find.

As a general rule stuff at the end of a long or complicated line will tend to be ignored. If it's important it be seen don't stick it on the end of a line.