Yes. It is possible to use less than 3 ml of undiluted or diluted mouse blood. The procedure is scalable. But as the blood volume is reduced, the size of the centrifuge tube must be reduced accordingly. The same ratio of blood (undiluted or diluted) should be used compared to the amount of Histopaque used.
Some users have reported that the procedure can be performed in microcentrifuge tubes. No studies have been performed to determine the minimal amount of blood that can be processed. If the volume of blood is less than 1ml, it might be possible to view the mononuclear cell band - or not. The most common sizes of microcentrifuge tubes are 1.5ml and 2.0ml. There are also larger 5ml sizes. If using a 2 ml microcentrifuge tube, would suggest 750-800 microliters of blood to 750-800 microliters of Histopaque 1083 or other appropriate density media.