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Two DC ties to the Eastern Grid, and one (some sources say 3) with Mexico.

Correct. ERCOT can send over 1,000 MW out-of-state through those connections. More on that at the bottom.

One of the Eastern Grid ties is through Oklahoma, and was briefly used for a few hours back in the '70s. It was not a grid-wide action, it was just one utility doing it. Depending on who you believe, it was either somebody going rogue just to prove they could connect in, or it was a proof-of-concept in case of emergencies, or it was because of some weird regulation, which may have been related to emergency testing. There was a big stink about it, and it almost brought the feds into play.

The grid in Texas used to be split into a northern and southern part, and I think each one had a tie into the Eastern Grid, possibly left over from WWII, and they kept those around after the two parts merged into what we know as ERCOT today.

As for the Mexican ties, those exist and I've also heard there are three, and they've been used when we've had problems. I honestly think we have all 5 ties to the Eastern Grid and Mexico in case it becomes profitable enough to start selling power out-of-state. Yes, we've used the Mexican ties when we've had problems, but I think they exist for financial reasons and were used because they were there.

The reason why I say that is that there is a project, the "Tres Amigas SuperStation", meant to connect all three US grids in New Mexico, and allow the buying and selling of power between the three. Texas could make quite a bit of money in the long run. The problem is that the project hasn't started (as of last year), even though it's been planned for 6 years, and it relies on some new technology that may not be present just yet.