There isn't just one answer to this. It depends on the context of the situation.
Geographically:
The junction of multiple mountain ranges is called a Knot.
The central block of mountains is called a Massif
The junction of three or more ranges is called a Confluence of ranges
*These often refer to the visible junction of the mountains.
Geologically:
If it is a junction between three ranges, it is often called a triple junction.
The junction of mountain-building zones or where tectonic plates meet is called an Orogenic Junction.
The junction due to continental collision is called a Suture Zone.
*These refer to what is underground
Geographically, Knot is the most used term
Geologically, Triple Junction is the most used term.
