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I have a sensor which is providing output corresponds to 0.3 milivolt and we need it to convert into volt raange. How can we do that ? which op amp can help ?

Try the circuit at

You will need Rf/Rin = Vwanted/0.003

If the signal is from -0.003 to +0.003, you will need an OpAmp which can output -Vwanted to +Vwanted. You would also need to supply -(1+Vwanted) at Vss of the OpAmp and 1+Vwanted at the Vcc of the OpAmp. (The 1 in extra is to be used for OpAmp which are not rail-to-rail).

(Or to add a small DC offset to the signal so that the signal seen by the OpAmp, but that DC offset will also be amplified.)

You should be able to get a lot of details in the textbook of this site (from which the circuit is taken).

3000 gain would ne needed but that is hard to achieve with opamp i think ?

If it is quasi static, it is in the doable domain, with one or with a couple of OpAmp in series, but the factor of 3000 will also be applied to noise in the signal. In short, you will have more problem with the noise than with the OpAmp, imvho.You may also have to fight against the DC offset of the OpAmp.

Another solution is to digitalize the signal with the use of the ADC (Analog to Digital conversion) of an MCU which will then output the numerically multiplied result through a DAC. The voltage ranges are easy to satisfy (if they are only in the "positive" voltages, and if the limited number of bits delivered by the DAC can be applied against a limited range like 0 to 5 mV, instead of the default range, set by default to a much higher upper limit of voltage). But very few MCU have a DAC (Digital to Analog converter). Basically, you use an MCU to simulate a low cost oscilloscope preset at a fix voltage range.

Note: the upper default ADC voltage limit is useless. Say it is 3.3 volt, while your max is effectively 3 mV, that is, 1000 times lower. That is 10 bits of resolution from your ADC lost for nothing. So, your ADC should be able to get a reference voltage (that you must supply) against which the bits of resolution from the ADC would be of some use.