Question

how fast should a 250g mouse run so that it can have the same momentum as a 3ton (1metric ton= 1000kg 1g=0.001kg) can a mouse realistically attain this speed yes or no?

Answer 1

Momentum = (mass) x (speed)

You haven't mentioned the speed of the 3-ton mouse.

Let's say that Jabba the Mouse is running as fast as he can ...
1 centimeter per day = 1 meter / 8,640,000 seconds.

Then his momentum is  (3,000 kg-m/8,640,000 sec) = 3.472 x 10⁻⁴ kg-m/sec

For Jerry the normal-size mouse to have the same momentum,

         (0.25 kg) x (speed) = 3.472 x 10⁻⁴ kg-m/sec

Divide each side by  (0.25 kg):  Speed = 3.472 x 10⁻⁴ / 0.25 m/s

                                                             =  1.388 x 10⁻³ m/s

                                                             =  1.388 millimeter per second.

A normal mouse can definitely attain this speed yes.
Even faster if he is startled or scared, or pursuing a girl mouse.