Okay kiddo, so imagine you want to go from one place to another in a big city. You can drive a car, take a bus, or use something called a train. Now, there's a place called Scarborough which is part of the city of Toronto in Canada.
Line 3 Scarborough is a train that runs through Scarborough. It starts at a place called Kennedy station and goes all the way to Scarborough Centre station.
The train moves on a set of tracks that are kept separate from the roads so it can move faster and not get stuck in traffic.
Lots of people use Line 3 to get to work, school or to meet their friends. Some people call it the "Scarborough RT" or "the SRT" because it's not a regular train that runs on wheels, it's a type of train called a "rapid transit" or "light rail" train.
That's Line 3 Scarborough in a nutshell, hope that helps!