You'll need to make up some kind of jig, to suit the height you want to lose so that you can cut the leg at the precise angle:
An ordinary hand saw will be fine if you take your time.
Thanks Freddie. I should have said that I am able to remove each leg from the chair. But take your overkill pointNot tricky at all, just three bits of wood the right height, it doesn't need to be as posh as the one in the pic. If you don't have the tools to cut some wood to the right depth then you could use three undersized bits of wood and screw a few screws into the bottom and adjust their depth to get the height exactly right. Chopsaw is complete overkill and I imagine nigh on impossible to get a single leg in to cut it anyway. No need to overthink it, they're only chair legs.