I am now a bit confused whether "sure" option with -reg3- is actually
doing an IV when I estimate it with endog() and exog() options.
Please read the documentation. The option sure on reg3 specifies the
estimation method. If you say sure, you are asking for sure, and sure
involves only OLS (strictly, GLS) techniques. It will not do anything
like IV. Options 2sls and 3sls will utilize the IV estimator.