Overdue notices will not send if the first overdue trigger lacks a delay value (i.e. if the delay1 field in the overduerules table table is null). It doesn't matter if your second and third triggers are set with notices and values. If you have a notice and no value for the first, that's it - nothing. This behaviour has a certain logic to it, I suppose, but as Katrin has pointed out, it could perhaps fail nicer and print a message saying there are other triggers set but they can't be reached since the first one is null.
Still valid.
*** Bug 11892 has been marked as a duplicate of this bug. ***
I believe it's no longer possible to configure a null value from the GUI, see also bug 25049 (and verified in testing). I think best to focus on bug 25049 now and add good error messages there.