Triple Vertex Parenting. How?

I have a modeled head with a separate object “scalp” with hair. It is scaled slightly smaller than the actual scalp area of the head object. I read that I to triple vert parent the head to the scalp (or vice versa).

My question is how do you select 3 verts on one object to triple parent them to another object when you can only select verts in edit mode on one object at a time?

Do I need to join the two objects first? Problem is when I skin the rigging to the head, the scalp hair strands disappear.

never heard of 3 V things before ?

but you can select one object then select another object go into edit mode and select one vertex then ctrl-p to parent vertex to first object !