My ac compressor will engage and then after 3 seconds disengages

Low freon charge .. That's what the 'low pressure' switch is for, and it is working! ..*Possibly* poor connection at the low pressure switch, wiggle/twist wire connector to see IF compressor re-engages.

If you changed the switch you would have evacuated the system and recharged it. Does it still have charge today or did it leak out?