He should run.
He can swim 3 miles in an hour and a half (3/2+1.5)
He can run to the other side in less than an hour.

The circumfrence of the circle is diameter x pi (3.14)=
3 X 3.14 = 9.42 MILES
Since he is only running half the circumfence, = 9.42/2=4.71 miles
He runs 4.71 miles at 5 miles/hr = 4.71/5= .942 hrs

Of course, if he really wants to save ime, he can wait till winter and run across the ice: )