Help bring me up to speed. I have two HDS Gen 3 sonars on the boat. I am thinking about the new ActiveTarger Live Sonar. Looks like the new ActiveTarger Live Sonar is not compatible with my Gen-3.
Is the HDS-Live the model after the Gen 3 or were there others in between?

So to upgrade to the ActiveTarger Live Sonar, I would need to upgrade to the HDS Live or the new Elite FS?

Yes, or find a used Carbon model.

Use Carbon or the new Elite FS

Since you are used to HDS units I would recommend an HDS Live there are rebates on the lives right know till end of March. Resales on HDS are very good so get ride of one HDS G3 and you can network the Live and G 3 together, The cost between the FS and Live with Rebates is worth it to get better performance By Staying with HDS Models The FS has one Ethernet port and you would need to by a nep box to have enough capacity to network besides use the Active Target. HDS Lives 7 have one port 9 and 12 have 2 ports.