1- Does Apple have any app that can make the iPad3 work as a normal GPS in a car or a boat?
Apple does not, but there are several third-party companies that do.
2- The built in GPS system is Satelite based or does it uses other technology?
All GPS is based on satellites. The iPad will use cellular triangulation where available to speed up location, but a cell connection is not necessary.
3- Is it possible to connect an external GPS antena to improve the signal?
Not to the best of my knowledge, no.
4- Does the GPS work withough internet access?
Yes, unless you're using an app that doesn't download the maps (the included Maps app, for instance, does not download complete maps and so requires an Internet connection to function properly).
5- Does the GPS work withough a SIM card / Mobile phone service?