Hi, can you suggest any simply diagram of a BJT astable generator, supply from 3V and capable to gen wave from range 500KHz to 2MHz.

 

The frequencies are high is problem number one. You need to pick the right transistors.

The Voltage is low but anything over 1.5Volts is not too difficult.

You already answered your own question by specification of an astable.

Modified Astable to more closely match your specs.

 

This one is about 2 MHz use some type of trimmers to tune the frequencies.

 

But in this circuit I need change two capacitors if I want to change the frequency

 

You want variable potentiometers on the base resistors.

Basic RC time constants.