I found that automated triangular (and 4, 5...) arbitrage was possible in the past, mainly due to fee-raking exchanges providing superfluous instruments. Opportunities were usually small, counterparty risk and fees usually large.

There also seemed to usually be someone else's bots doing the same thing, although occasionally decent sized trades appeared in slow/extraneous markets. Exploiting these mechanically eventually results in humans looking for the source of the arbitrage, and even if there aren't competing bots running it won't last long.

Arbitrage based on first-moving markets and slower moving markets was pretty common, and some people seemed to have automated it.

And of course the market making robots run by the exchanges are the most fun to watch. Lots of times they trigger the crossover bot traders' algorithms on purpose.