a) dur='PT3S'
b) dur='3'
c) approxDur='3'

LB> Is any of them TEI conformant?

Presuming your modification is properly documented, they are all
conformant.[1]  Option (b), however, does not follow TEI recommended
practice.[2]

Thus, if I had a really good reason to stick with (b), the current
encoding -- e.g., very expensive software that knows how to process
that, but not the others, the way I want to -- I'd use it. Otherwise
I'd prefer (a) as best, (c) next.

Note, however, that I am applying P4-like definitions to what is
ostensibly a P5 document, which Council hasn't actually decided to do
yet.


Notes
-----
[1] http://www.tei-c.org/P4X/CF.html#CFDOC
[2] http://www.tei-c.org/P4X/CF.html#CFREC