Just to be clear, Verizon is not changing plans again or eliminating it. They are raising the cost of your current plan.
Do you know how much data you actually use for those 3 lines? The 3GB for the Mifi is marginally cheap to add to Verizon's current pick you size plans. When we had 2 smartphone lines with unlimited data, I think we paid about $150/mo. That's about the cost of the 18GB plan plus two smartphones today. Add a 3rd smartphone for $20 to share. I suspect that 6GB per line per month is plenty for most people that do not stream audio/video all day. Add the Mifi for $10. Probably less than you are currently paying for all the services, and you get unlimited texting and voice minutes.