It will be functional with just 2. Yes it might be a little more sturdy with 3, but you could always say it will be more strong with one more screw. 2 screws will hold it fine and keep the clip from rotating so I would just use it. If something ever happened again then you could tap all 3 screws and put bigger ones in. Do you know how it got stripped? I would guess you got a screw cross threaded? If you keep the screws snug then stripping the liner shouldn't be a common problem.
Tapping a piece of metal isn't that hard but you have to have the right tools. You need a tap that is the right size for the size screw you plan on using. Screws are sized by their diameter and number of threads per inch. Once you find some bigger screws and a tap to match it you then drill out the existing screw hole with the size drill bit the tap specifies. You then run the tap into the hole as straight as you can which cuts new threads into the liner. Then re-assemble with your bigger screws. Unless you already have the tools, which it doesn't sound like you have, it would probably be cheaper to send it to someone to have the work done.