How to make 3 way calling

If you are referring to Cellular Voice Calls then...

SMS, MMS, WiFi calls, Call Forwarding, Call Waiting, Call Merge, Conference Calls, hotspots, and voice mail (Visual or Normal) all are Carrier Specific features. Contact your Carrier (Cellular Service Provider), the agency that issued the SIM / eSIM to you.

If you are referring to FaceTime calls then...

FaceTime group call

During a FaceTime call, tap the screen to show the FaceTime controls (if they aren't visible), tap. at the top of the controls, then tap Add People. Type the name, Apple ID, or phone number of the person you want to add in the entry field at the top. Or tap. to add someone from Contacts.

Make a Group FaceTime call on iPhone - Apple Support (IN)