As a couple of others have posted, I would pick up and start over. During my walk back to the holding blind to start over, I would develop my training strategy, starting with WHY did the dog fail to pick out that third mark?
Then, back to the line, where I control everything. Point out each gun station, que with "mark", then signal for EACH mark separately. If my dog doesn't turn for that last mark, I will MAKE her turn, because while I was at the holding blind, I put her collar back on her, and there is a short piece of check cord dangling from it. This is TRAINING. I would turn her physically, remind her "mark", and not signal for that last mark to be thrown until my dog signaled to ME that she remembered ("Oh, yeah, there's another gun station out there"). Noise at the gun station helps.
Every dog is different. If this was, indeed a senior-level dog, not ready for triples, then WHY run her on the marks as a triple in the first place? If the dog had the triples concept but had never run this big a test, run the memory bird as a single, followed by the other two as a double, then run it back as a triple i.e., a teaching triple.
Lisa