Originally Posted by ItsJustMe
Overly complex. Assuming the swim speed is 100% in the direction from one bank to the other, it takes them 1m / 2 m/h = 1/2 hour to swim across. In that time, the current will carry them 3m/h * 1/2h = 1.5 miles downstream. They will reach the shore 1.5 miles downstream from a spot directly across from the spot they started in.
Rivers flow faster in the middle than at their banks. Is the flow speed at the bank or in the middle? If it's at the banks, the river will flow faster down the middle and the swimmer will get carried further downstream while crossing the middle.