The Three mast I use is shared with EE, their service was off for the same 3 to 4 days so the fault must have been the power supply.I used to get really good speeds from EE but Three was cheaper and good enough. When Three stopped working well, I tried EE again, and they were terrible.
Three and EE had been using the same mast, and it seemingly wasn't there any more.
But it has come back on line for Three, and with it, decent performance.
I've not checked to see if EE also has it working again now.