Will the rbs50y or any other orbi outdoor mesh extender work with my ac3000 system that already has 3 satellites connected indoors?

Yes, the RBS50Y is in same series and will work with the Orbi RBR50. Yes, you can connect more satellites, but remember they share a backhaul circuit. If you can hard wire any of them back to the router, that would help performance. I have two that are hard wired and one that is wireless.

what would you say is the additional coverage outside the house? how far from the extended do you still get the wifi

I just added the outdoor Orbi to my network. Claims that it can add up to 2500 sq ft outside seem fairly accurate to me. Prior, I lost my WiFi signal about 15-20 feet from my home with 2 satellites attached to my mesh network. I am now receiving a strong signal approximately 50 feet in all directions from my house. A little further, say 100 feet, in my backyard where I installed the satellite. My only criticism is that the plastic casing seems a little “cheap”, considering the cost and the fact that it is supposed to be “all-weather”. The satellite is also larger than I anticipated. I wouldn’t completely rely on the included base stand. It is very lightweight and feels top-heavy when the satellite is attached. You would be better off mounting it to the home or a fixed structure.

Will I be able to extend a signal from my access point?

Mounted mine on side of my garage on the wall under the eve in sight of the base router, 50 to 60 yds from base. With it my Wi fi signal strength, Upload and download speeds are almost the same as in the house next to the base router. download 239.00 ,mbps. upload14.75 mbps Without the extender I didn't have enough Wi fi signal inside the garage to get online. I am happy with it. does what it is intended to do. Works better than ethernet cable I had run to garage before with faster speeds.