iMac 3 in 1
Starts up and then shut down
Question marked as
Likely a hardware issue. You haven't provided us with any information and frankly I have no idea what a iMac 3 in 1 is as Apple has no such machine. I would recommend first unplugging the power cord from the back of the Mac for 2-3 minutes then plugging it back in and holding down the power button for 15 seconds or so. If it does not startup then you have some decisions. If the computer is older than a 2017, your best option is simply to replace it as it is obsolete meaning parts are difficult to obtain and it will not run current versions of Mac OS. If it is a 2017 or later, then please take the computer to your local Apple Authorized Service Provider to have it worked on. If you don't know of one, you can use both:
Apple Authorized Service Provider Locator (AASP) & Genius Appointment
or you can call AppleCare. AppleCare Contact Info
Good luck.