Are there any real differences between different generations of Helix? What's are the main differences?

Yes

Linkabilty

Every generation of Helix 8, 9, 10, 12, and 15 is linkable via Ethernet.

I am 74 and have a helix 8 g3n up front and a 2 g2n at console How do I pair them? Also, I and going to get another just for mapping what do I need to be able to view all three on my 12 unit?

Helix models starting in the second generation that have n in the generation name are Networkable. That was done since Helix 7's are available as non-networkable or networkable.
You share data via Ethernet network.
Each Helix display needs the AS EC QDE Ethernet Adapter to connect an Ethernet cable to it == it's because of the quick disconnect cable feature.
An Ethernet cable has two ends so that is two devices that can be connected together with a single cable.
To have more than two devices networked requires an Ethernet Switch (hub) AS ETH 5PXG. Then each device would connect to the Ethernet Switch.

Helix units do not share any mapping of any kind with any connection, they will share waypoints, sonar, and Ethernet devices.
If you want the same mapping on all units, each unit would need a matching map card.

Second and third generation Helix units have different 2D sonar standard frequencies.
Also there is more to discussing sonar units, it's the exact model they are which will determine what they can share.
There is the CHIRP model which is 2D sonar and GPS.
There is the DI model which is 2D sonar, Down Imaging, and GPS
There is the SI model which is 2D sonar, Down Imaging, Side Imaging, and GPS.

All three of those models can be a GPS/Map unit without a transducer connected. The CHIRP model would be the lowest cost and a CHO (Control Head Only) model would be even a lower cost.

Wayne P. Thanks so much, this will help me when I connect these unit together. Have a Great day and good luck this fall.

I have a helix-10 that I use for navigating and SI looking for structure and a helix-15 on the bow for using mega live while crappie fishing, should I run an ethernet cable between them to network? I'm not sure I really need to, or what the advantages would be other than sharing way points.

Since you did not state which model of Helix 10 and which model of Helix 15, this is general:
If the H10 is a third or fourth generation, when you network them, you can share their data with each other including displaying MEGA LIVE.
Sharing waypoints would be like navigating with the H10, mark a waypoint with the H10, and that waypoint being displayed on the H15. The waypoint will not be written to the H15's memory, just visually displayed.

If you have a Minnkota trolling motor with iPilot LINK you can have motor control via the display and have more memory of Spotlock waypoints and iTracks. Plus have Follow-the-Contour with Lakemaster HD mapping, Auto Chart mapping, and Auto Chart Live mapping.

Humminbird HELIX 10® MEGA SI+ GPS G4N
Hummingbird HELIX 15 MEGA SI+GPS G4N

I do have minnkota trolling motor with ipilot link as well, so you would suggest running the ethernet cable and start networking? I'm new to all of this.
Thanks for the info

MEGA LIVE, iPilot LINK, and a display are all Ethernet devices. Helix units have a single Ethernet port. The only way to network all of them is via the AS ETH 5PXG Ethernet Switch. Then each device would connect to the Switch. LIVE and LINK come with an Ethernet cable so you would need two more Ethernet cables for the two displays. After all of the devices are networked they can share all functions they have except mapping. When the trolling motor is deployed, you can use iPilot with its remote Heading Sensor as the GPS/Heading source for the bow display to help with positioning on waypoints like offshore brushpiles when fishing them. If you were to add Target Lock, that would connect between the MEGA LIVE and the Ethernet Switch/power so it would not need a separate Ethernet connection or power connection.