Hi btbrad,
Welcome to our forum and thanks for contacting us with your question.
There are multiple ways to achieve something like that:
1) You can remove charts' spacing and hide separating axes.
Demo:
https://jsfiddle.net/BlackLabel/5bom6k9z/
2) Use single chart instead of three, and add additional axes like in this stock chart example.
Demo:
https://jsfiddle.net/BlackLabel/4snqumkf/
3) The last and at the same time most complicated option is to use SVGRenderer tool to draw line that would extend the crosshair or replace it completely.
API reference:
https://api.highcharts.com/class-refere ... VGRenderer
Let me know if you have any further questions.
Regards!