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The "pull down"
resistors hold the
cathodes of the diodes
low, making the diodes
forward biased. The output at the anodes
is held low, Tr1 base is
low and its collector
high.
The base of Tr2 is high
and its collector low.
A and B and C must all
go high together, to
reverse bias the diodes,
and make the base of Tr1
high.
The collector of Tr1
goes low, together with
the base of TR2.
The collector of Tr2
goes high.
The output has gone high
because A and B and C
have gone high.
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