Una no es ninguna

Una no es ninguna
 

At a social they passed around a plate filled with small cupcakes, from which I took just one and ate it. The host urged me to have another, and even more, with something that sounded like [hope I got it right]:

Una no es ninguna, dos apenas la mitad de una, tres es una, y volvemos a empezar.

Followed by a hearty round of laughter. Reckon I should have asked for an analysis of that since the math didn't add up, but the social was going strong and I didn't want to slow things down for a grammar lesson there.

So, did I hear correctly? Is it a commonly-used expression?

Yes, and you wrote it well. It is just a play of words that urge you to eat more than one and keep on eating till your full. :)

Quite a common saying, mostly for being invited a cup of wine or other kinds of alcohol. :)

In central Mexico: Una (copa) no es ninguna; dos, ni la mitad de una; tres, apenas una, y como una no es ninguna... ;)

However, if you're looking for the maths supporting this, you may think in fractions... except for the third cup. :D