Yep. 73mm mountain crankset will work fine in a 68mm shell. The mountain bottom bracket usually comes with 3, 2.5mm aluminum spacers. Place one on each side between the cup/bearing and the shell. I have had several bikes configured that way at various times.
Concerning the 3rd spacer: It may/may not be needed if you are using road bottom bracket in road shell with mountain crank. The hollow tube inside might need to be mix-n'-matched though. At least that has been my experience.