I don’t have a Tracker 3 but I’ve played with one and am familiar with it’s operation. FWIW, I am pretty handy with my Tracker 2 as well.
RE: Switching signals – I have two theories. . .
(1a) After one minute in signal suppression mode, Tracker 3 reverts to regular mode. Perhaps you simply timed out. (1b) Tracker 2 would display all signals found, then hone in on the closest (while auto-suppressing farther ones). Perhaps Tracker 3 does the same in regular mode?
(2) A simple push of the ‘options’ button suppresses a signal: (“SS” appears on the screen briefly), then displays the distance & direction of the next closest signal (for one minute). A long push-and-hold goes to ‘Big Picture Mode’ which cycles through all signals it receives. Perhaps you entered ‘Big Picture’ mode? That seems like something I might do given my experience with the Tracker 2.

RE: 3rd burial – Spurious emissions happen all the time near my house in a city: television sets, car engines, cell phones, funky lightbulbs. They even happen on my local resort b/c the mountain is full of metal— buried conduit and the rock, itself, is very ferrous— which propagates RF signals unpredictably and can result in sustained, mysterious “3rd signals.” Tracker 3 is really sensitive and picks up on all kinds of stuff my Tracker 2 doesn’t. Try going out to a large field (40 m away from anything) and see if they still happen. Another option is to find a really fancy beacon (like an Ortovox S1) which can run a diagnostic on your beacon— but I think the diagnostic only tests your transit function. I’m not sure. Lastly, get in contact with BCA. They’re a solid company which will take your beacon in to see if it’s broken.

It’s good you are practicing so you know how it behaves in different environs and can learn to ‘see through’ the noise. On my Tracker 2, I can usually tell a signal from spurious emissions by the length of the signal on the display. Beacon signals broadcast twice a second for a specific duration, twice a second.
In a multiple burial scenario, the sound of each of the signals broadcasting creates a particular rhythm for that particular scenario: “beep beep. . . .beep beep. . . .beep beep. . . .” (for 2 signals) or “b’deedit. . . b’deedit. . . b’deedit. . . ” (for 3 signals). Spurious emissions (mysterious, 3rd signals) don’t fit the rhythm of scenario and have a shorter duration than true beacon signals.

I hope this helps!