Using an old Canon 100mm FD macro lens on a modern Sony a7III

Yes, it works when you use a Canon FD to Sony E adapter which can be bought for a few bucks on Amazon. The lens is a manual focus lens and the adapter will not change that. Since you are about to make some macro photos, you should learn to use the manual mode anyway. It’s the way to get the best results. The Sony supports you with a magnification function which is awesome for any manual and macro lens.

BUT

There is a downside to this. You will face some rather special bokeh. I hope you like hexagons. If not, you can still get the Sony 90mm macro for 7 times the price. Have a look for yourself.