Radobojski - there are three ways to achieve this... I am going to use three flashes as an example....
1st - using the hot shoe to trigger the other two flashes... which ever brand you go with... there is a manual mode that will trigger when it senses a flash from your camera... usually to dull the flash strength off the pop-up... I use a gary-fong puffer... on the hot-shoe it defuses the light enough that it creates a nice fill affect... does not matter what camera you use... it works... even use it with my Hasselblad H systems....
2nd - using actually three flashes... one on the camera as the main source... and two on manual as before... the flash on the camera can be used as a bounce light which gives you a room fill affect... or my favorite I add a ring light adapter to it... and give it a specific purpose...
The issue with this... is you loose light at the bottom of the ring... I corrected this by pulling the whole thing apart... and then using HVAC metal reflective tape all over the inside... increased the brightness and gave me a full ring flash... easy DIY project...
3rd - is using off camera triggers... again... the flashes will be on full manual I do not think there is a P-TTL set yet... I went with iShoot series and got them for Nikon - just because they will be easier to resell later... they are fine wireless triggers never had an issue with them... I personally can't use it over 1/125 or 1/160 speeds... so usually not what I want to do... unless it's a highKey style shoot...
All these systems work indoors and outdoors... but I found one thing lacking... power... I have since switched to Strobes... which is a whole different issue all together...
I hope this helps you out a little...
Tony C.