I have no knowledge on Pascal, but, after reading up from here, I noticed the following:
A program in Pascal starts with the reserved word 'Program' (although it is not explicitly required) and ends with 'End', following a full stop (this is required though).
After implementing the above advice, I had compiled it through OnlineGDB once again, and it compiles and runs well.
Woud you believe you forgot the full-stop after the last end? Supposed to be "end."
var N1, N2, N3, Sum:integer;
begin write ('please enter the first number');
write ('please enter the first number');
write ('please enter the third number');
Sum:= N1 + N2 + N3;
write ('you carefully calculated answer', Sum);