Air condition compressor clutch comes on & shuts off in 3 seconds. Comes on again about 20 seconds & shuts off in 3 seconds again. Does not get cold in vents.
This is a typical operation of the air conditioning compressor clutch when the R134a refrigerant is low. Using a set of air conditioning gauges to check the pressures and using a leak detector is advisable. When the low side of the gauges goes below 30psi the low side pressure switch disengages the clutch.
Any sign of a slight oil discharge from any hose connection or joint, compressor front seal, evaporator or condenser would be a sign of a small leak. Putting the leak detector in the A/C vents when the system is running and the fan is on can help determine if the evaporator is leaking.
If no leaks are found then recharging the system to the proper pressures should fix the problem.