If you can micro all 3 probes at the same time with the rest of zoo engi stuff, then you are probably hacking. Even with 1 probe zoo engi used right is pretty micro intensive, to use 3 at once you would have to ignore all your other abilities. You would have to deselect your engi (no reloads, no kiting, no running for your life) and trust that he is OK to cycle through the probes. Sure they are helpful for like 5 minutes, but after that the extra 2 probes are hardly useful. Also the slow and armor debuff are not that great compared to plasma discharge, roughly equivalent in those stats but discharge is so much more useful and is so much easier to use. Using both at once is silly. The T2 upgrades are so much better than 2 more probes. Probes themselves don't need a buff, but their upgrade does. A second and third probe is hardly useful. I would get everything else in the tree (and then go to other tree) before that, hands down.