I like how confidently you spout yiur 731IQ BS here.
Lets breakdown it logically:
"Atleast three", the tweet screenshot yiu posted literally said both pilots ejected from twin seater F/A-18, hence MB mentions lives saved, not aircraft lost.
So, logically yiur statement should be like atleast 1 aircraft might be lost with both pilots safe.
Next bunch of words are "Western fighter jets",
unless yiu think JF-17 is a western fighter jet to honor NG for starting Super-7 project it does not make sense logically.
Next bunch of words are "were shot down. The details of the crashes of 7785, 7786, and 7787 have not been disclosed."
Again repeating myself, MB does not care about aircraft loss via shotdown, accident, friendly fire etc and not even ejections just lives of pilots saved hence the first screen shot yiu shared already has pilot 7787 and 7784 from F/A-18 twin seater included as mentioned by MB saying both pilot were safe.
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This is not new, yiu confidently shared the J-10C deal of PAF leaked by Indian hackers as released by Pakistan

last time and have not replied to me yet when I corrected yiu there. I wonder why?
As I replied above, MB cares about lives of pilot it saved not ejection or crashes or accidents. Think logically even if successful ejection happens there is no chance pilot will survive it. So, how will MB know to claim pilot is safe unless the transponder you speak of will even share pilot vitals to MB for them to know, which I think is not allowed by any airforce on Earth or possible reliably to know.
To answer your question about how does MB know, well they are informed after a successful ejection and pilot is safe by the airforce/navy that on so and so jet your so and so model MB seat worked perfect and our pilots are safe etc, which the MB will put out in their press release. They even have a watch exclusively for pilots who have successfully ejected.
MBI
martin-baker.com
There is cemented carbide chong (M H Yang) in PeeDF, even he mentioned something like "that lives lost is biggest loss than equipment loss" but pakees of PeeDF are like nothing happened to saab erieye but some how 5/6( as per sindh CM) PAF personnel were crushed under hangar debris, 40 odd has injuries. Seeing this level coping that chong shocked

.
But our resident hanese is different it looks like.