Blocks are numbered one, two and three. Snipers are lettered A, B and C. The path you take to reach each step is dotted in blue.

First push block 1 to the north west. Make sure that for the last couple of steps sentry C has his back turned, else you'll get shot. When block 1 is in place, duck behind block 2 as shown.

Now we move block 2 to the NE. If you want you can wait until both sentries have their backs turned, jump up, push the block one space North, and then duck back (as shown by dots) This will get you one space closer, but I don't think its absolutely necessary. Either way, now wait until both sentries are turned, and push the block northeast. Jump back (as shown in pic) to avoid being shot by the now watching sentries. Move block 3 against the west  wall, and move block 2 against the east wall. Now you're done.