New fuel may simply soften the old fuel which would have turned to gunk over the 3 years timeframe. So, adding new fuel and fuel system cleaner will simply softened gunk where it will move into the injectors and clogs them up (if they aren’t already clogged up that is)...
As is, I think the fuel manufacturers recommendation are not to store fuel within a vehicle for more than 6 weeks
After such time sitting in the tank, the safest and best approach is to drop the tank and clean it of all gunk and rust... same for the rest of the fuel system... Doing such seems like it will take longer but usually it’s the quickest way to approach the problem of very old fuel in the system.