Is it possible to have raw ngs data to practice
At least PacBio do provide some example datasets on their DevNet resource. You should be able to get a lot of different Illumina data from the public data in BaseSpace. I am not sure where to look for official samples from Roche 454, but I wouldn't bother too much as it is discontinued anyway. Rather go for MinIon or IonTorrent data, if you want more.