I recharged it but it only lasted for 3 days. What do I need to check/replace so that I won't have to keep buying recharges?

 

Obviously there is a leak.

A dye can added to the Freon system which glows under UV light to show leaks. There are also Freon detectors the when passed over a leak will beep.

The AC system has lots of pipes, hoses and other components, some are hidden under the dash.

A few things to check are the drive shaft seal on the AC compressor housing and leak in condenser in front of radiator due to road rock hitting at tube.

If you find the leak might replace the dryer as it may no longer be effective due to air (moisture) leaking into the system.