convert f1 to h3 and stamping outside the us

I am currently on OPT untill sept 30 unfortnately i missed the H1b this year. . Is there a possibility that an F1 visa can be changed to H3 visa? My emplyer will be working with an attorney moreover, after approval of H3 do i need to go for stamping right a way or i can start my training with h3 stanp

your support and guidance will be much apprecitaed

Well, I do not know much about H3. Please work with Attorney.