According to analyst estimates, the 3G version of the Apple iPad tablet, which hit the US store shelves late Friday, probably sold around 300,000 units – pre-orders and online sales included – during its debut weekend.

In a Sunday note to investors, Piper Jaffray analyst Gene Munster said that the estimated first-weekend sales figures of the Apple 3G iPad were largely based on the information forwarded by 50 Apple stores round the country, as well as checks with in-person attendance of the device’s launch in New York and Minneapolis.

In case Munster’s statistical assumption is correct, the iPad 3G – starting at $629 - evidently sold as many units in its first weekend as its April 3-launched $499-priced low-end Wi-Fi-only counterpart sold on its first day.

Moreover, with 40 out of the 50 surveyed stores reporting that the newly-launched 3G iPads were sold out by Sunday, Munster is of the opinion that the reason for the sellouts is the stronger-than-expected demand as well as a lower-than-projected supply.

Munster said in the note to investors: “Near-term, this may put downward pressure on launch day/weekend statistics, but long-term we see it as a positive, as consumers are definitely interested in the iPad as a new category. In the first several quarters, we believe Apple will sell about 60% wi-fi only iPads and 40% 3G models.”