Something

int main(void)
{
	printf("\nPlease type in a sentence or sentences and hit Enter.\n\n");
	
	char c;
	
	while ((c = getchar()) != EOF)
		
		if (c == ' ')
			putchar(' ');
		else if (c == '.')
					putchar('.');
			  else
					putchar(c + 3);
			
	return 0;
}