sample.pgc:10: ERROR: syntax error at or near "My_Proc"
General usage questions regarding PostgreSQL and ECPG should be sent to the -general list.
I don't know the correct answer here but maybe using dollar quoting within the PostgreSQL SQL statement instead of double-quotes will help. Something like:
prepare prep_stat3 from "call $$My_Proc$$()";
Asking an actual question does help others to figure out where you are confused instead of just guessing.