Question

What is radius of earth to act as a black hole​

Answer 1

One of the terms used with black holes is "photon sphere", the radius of the orbit of light around the black hole. For 3 solar masses this radius is 13.5 km, 3/2 x the event horizon radius. The event horizon radius is also called the Schwarzschild radius.

Answer 2

Black holes are formed when huge stars die in a supernova explosion. They may be billions of times larger than Sun. Earth cannot act as a Black Hole.