Question

If you have one extra chromosome you have downs syndrome. What if you have two or three extra chromosomes.

Answer 1

You cannot have!!!!!
Zygote receive 23-23 chromosomes, in total, 46. Sometimes, it happens that due to few conditions, zygote receives either 45 or 47 chromosomes.
You asked correctly, but there is have been no such possible case where you have two or three extra chromosomes.
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