My bad. You said:
...I have fixed other electronics...
and I thought you know how a transformer designed.
These 3 wires is from 2 secondary windings and one of them is high voltage with about 2100 volts. Don't even think to touch them, especially when the microwave is plugged in.
There suppose to be another 2 wires on the other side of the transformer. These wires should come from the control board and the door switch. These are the wires which should be connected to the primary winding.