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?

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.

Neptune is the eighth planet from the sun. it was the first planet to get its existence predicted by  mathematical calculations  before it was actually seen through a telescope on sept. 23, 1846. irregularities in the orbit of  uranus  led french astronomer alexis bouvard to suggest that the gravitational pull from another celestial body might be responsible. german astronomer johann galle then relied on subsequent calculations to   spot neptune via telescope. previously, astronomer galileo galilei sketched the planet, but he mistook it for a star due to its slow motion. in accordance with all the other planets seen in the sky, this new world was given a name from greek and roman mythology — neptune, the roman god of the sea.

English can you write this in english do we can you