There are numerous sahih reports, such as this one collected by Bukhari, that the sunnah of the prophet was to perform his ablution in such a way. He would rinse his mouth and his nose with a single handful of water, that is to say he would take a single handful of water and rinse his mouth out with part of it, then rinse his nose out with the remainder. And, as with the washing of other body parts, he would do so three times (although there are reports of him only washing once or twice instead).
Thus, three handfuls of water (instead of six, if one used a handful for the mouth and another handful for the nose).
As for a definite explanation for this behaviour, I know of no evidences; following the sunnah of the prophet for no reason than because it's the sunnah of the prophet is often reason enough for most Muslims. However, given that using a single handful of water is less wasteful than using two, this can be understood as a general application of avoiding extravagance.
This point is brought home by a hadith collected in Sunan Ibn Majah (although I cannot speak for it's authenticity), wherein the prophet was asked about whether there could be extravagance in ablution, to which he responded, "Yes, even if you are on the bank of a flowing river."