Can somebody suggest me step by step how to connect 2 or 3 or 4 Bose Soundlink III Speakers with 1 Bluetooth Device let say iPhone whether I need to buy connecting accessories to do that?
Most you can do is pair 2 with an external bluetooth transmitter.
If you use JBL bluetooth speakers and their Connect plus app it lets you pair an unlimited number of their speakers. https/itunes.apple.com/us/app/jbl-connect/id994041762?mt=8
Bluetooth is like the traditional marriage - one to one. Unless Bose haven't designed something custom, you can't run more than one device off an iPhone.
Most you can do is pair 2 with an external bluetooth transmitter.
If you use JBL bluetooth speakers and their Connect plus app it lets you pair an unlimited number of their speakers. https/itunes.apple.com/us/app/jbl-connect/id994041762?mt=8