Why is my compressor on just for a few seconds?

Before doing anything, ohm out the compressor at the 3 pins. Common to start, run to start, and run to common. Common to start, plus Common to run, should equal close to what start to run reads. If there's a significant difference then there's likely problems with the internal windings.

But it sounds like a start component issue, most likely. I recommend checking the start relay (Use a multimeter to test the start relay for continuity between the start and run terminals. If the start relay does not have continuity, or if it smells burnt, replace it)

After that would be the start capacitor. If you have a meter capable of reading capacitors that large then check that. You can look up the horsepower rating of the compressor using the model number on the compressor data tag. Then get a 3in1 start kit. There will be 5 wires coming out of it. The black wire without a crimped on connector that is closest to the black wire with a crimped connector would be for incoming power. Neutral goes to the other. The 3 pins on the compressor are usually labeled. And the 3 in 1 kit also shows where to hook things up.