What tool should I purchase in order to replace the bottom bracket in my Giant Defy 3?
You don't need to remove or replace the bottom bracket to fit R7000 series 105 cranks (unless the bearings are won out of course). The Shimano SM BB72 bearings are Hollowtech II for a 24mm spindle so the R7000 will fit.

If you need to replace the bearings, special tools are needed to drive the press fit cups out of the bottom bracket shell and press new ones in. This must be done properly - the cups have to be driven out and pressed in square to the shell otherwise it can be damaged.

The tools are relatively expensive so it's a good idea to get a local bike repair shop to remove and press the bearings for you.