Looks like Russian planes don’t use always GPS (or their GLONASS). They are all over the place in the above pic.
Sometimes they avoid moving clouds. Do you have the weather layer on?
No, but that route is the normal one they do almost everytime
Frankfurt has pretty rigid noise abatement rules.
ATC won’t be able to clear them direct to wherever they can before they reach a certain altitude.
You can see the third guy climbed faster so got the direct faster.
At least that’s my guess 
Thunderstorm in the mountains and clear sky over the sea.
with technological sophistication getting better all the time, it can be very helpful in seeing and also learning the direction used
departing at almost the same time, will the three of them head to the same airport?
Or they are spreading the noise in different neighborhoods? Like they are not allowed to have noises back-to-back.
No. They are going to different destinations but the routes to Vancouver, San Francisco and Los Angeles are at least over Europe the same
Looking at these aircraft coming from north america to EDDF (Frankfurt).
Right before London, they line-up
Wow, very nice to see, like geese in a row 
Such phenomena often occur, as well as in Indonesia, especially at Soekarno-Hatta airport and Sultan Hassanuddin airport in Makassar
motorcycle or bycicle racers are doing it for using the slipstream.
I would not assume that this is the same reason for aircraft 
Perhaps they want to save a few gallons and drop it over Rheinland Pfalz 
Pretty sure not. Fuel dumping is a serious event and nobody wants to do the paperwork afterwards