It depends on your time, patience, if you have other rescuers to foster, and so on...
It can be done and chances for succeeing are good also at 3 years, even older. And very good chances if you are used to this work. But it will take lotsa of time patience etc. Half a year at least. It may take years too.
Some dont get pets, but they at least get half tame and manage to live indoors...
Unless he is already halfway tame being friendly withyou, or dumped home cat, it gets usually quicker.
Another quick window is if he is severely sick or wounded - and you help him. These cases can go astonisinghly quickly....a few weeks.
If he is managing well outside and or if you dont have the time and patience - it is of course entirely feasible to see he is neutered and let him be outside...Like you plan to. Very much OK!
Good luck!
