Microwave turns on by itself for 3 minutes as tho it is on a timer, but timer is not set

as you stated...microwave counts down in seconds.
300 seconds is 5 minutes 180 is 3 minutes.

Try setting your timer for 2.75 mins to 3.25 mins

1. Open door
2. Press clock/Kitchen Timer button the display will show "0:00" with the hour"0" flashing.
3. Enter the correct hour by turning the potary dial. Enter 1-12
4. Press clock/Kitchen Timer button the display will show "0:00" with the minutes "00" flashing.
5. Enter the correct minutes by turning the potary dial. Enter 00-59
6. Press clock/Kitchen Timer. The colon will remain flashing.
7. Close the door.

The first thing I would suggest is a hard reset. Unplug the unit for a minute or so. Then plug it back in, set the clock and try again.

If that doesn't work, leave it unplugged for about 5 minutes.

1. Hold the Timer/Clock button down for a few seconds till it beeps and shows 12/24 hours.2. Type the time into the display. Use the minute buttons to set the hour, and the second buttons to set the minutes. 3. Press the clock button again.

The first thing I would suggest is a hard reset. Unplug the unit for a minute or so. Then plug it back in, set the clock and try again.

If that doesn't work, leave it unplugged for about 5 minutes and try again.

Highly likely your microwave has "failed".
Down inside the microwave is a part called the Magnetron. This is the part, (about the size of your fist), that makes the sound waves to cook the food.
On some models, it can fail, whilst all the other stuff works.
3 possibilities:
1) You need a fuse replaced.
2) You need a capacitor replaced (That part "starts" the magnetron) or
3) you need a new magentron.
In any regard, very often people will not opt to have it repaired, because the labor & parts can cost more than a new Microwave.
God bless your efforts.

Its fried the machine time to buy a new one sorry

Pressing "timer/clock" button 4 times turns on and off demo mode.