My 98 Honda Accord Vtec 4cyl. eng is starting fine, but after start the idle dips a few times and regains for approx. 3min. If I drive without warming up the car will hesitate and the gas pedal will delay for the first 3min. After the initial 3 minutes the car drives normally.

Seems like the coolant temperature sensor is giving the computer erroneous readings, and it is not adjusting the fuel mixture properly for cold engine operations. You should have the check engine light on, indicating this malfunction. The idle fluctuations are normal. The engine is trying to die, from the improper cold mixture, and the computer (ECM), recognizing low idle speed, is sending it back up. Another possibilty is that the butterfly on the throttle body is not closing completely, to allow richer mixture needed for cold starts.