You can have 5 party lines on a digital and 3 on analogue extensions including the phone you use
on the digital phone make the first call, then press conference key, dial second party, press conference twice and all 3 will be talking. press conference again and then dial next party. press conference twice once connected. keep doing this until there is max of 5 party's on the call.
Analogue phone. make the first call then when talking to that party press r of flash, then dial next party and then press flash or r plus 3 to bring all party's together