It looks like the minimum required voltage is 5v. I don't know if this means you'll get a very sharp falloff in temperature below 5v, of if it will simply not work at all, but if you look at the resulting temperatures for 10.4cm at 5v, 7.4v etc you might be able to extrapolate some idea of performance ( assuming it works at all ) at 3v:
5v, 0.74A, 40 degrees C
7.4v, 1A, 75 degrees C
9v 1.1A, 91 degrees C
12v, 1.33A, 110 degrees C
Roughly you can see that you get about 10 degrees C per volt, with it showing signs that it might trail off slightly at 5v and below. I'd estimate 3v might net you 20 degrees C, which at room temperature is not far from being equivalent to it doing nothing.
... now I'm going to get really speculative and probably somewhat wrong with my maths here, but;
If you assume you're using a boost converter that can deliver about 0.74A at 5v and assuming 80% efficiency your 2.5Ah batteries would deliver maybe 2Ah. That's 2 amps for an hour, or 0.74amps for 2.7 hours.
So, my incredibly back of the napkin and possibly false conclusion is that you could use a boost converter and heat to 40 degrees for about 2 and a half hours.
Overheating is a subjective term; depending on whether you're worried about the pad itself breaking down, or whatever it's heating being damaged/burned.
If it can run up to 110 degrees C safely at 12v, then running at 40 degrees at 5v even in an enclosed space would probably not cause any problems to the fabric itself. If it's contacting your skin, however, or used to heat something that's temperature sensitive you might get uncomfortable quickly!
Although my calculations are terribly speculative and probably demonstrate my lack of understanding when it comes to powering from batteries, there's one thing to remember; generating heat from electricity is terribly, terribly inefficient. In fact, heat is usually a byproduct of some other desirable result. You might be better with good old fashioned combustion: https://www.zippo.co.uk/product.php?id=1492