Generally, unless you're actually a good trader or unless you irresponsibly invested too much and really need money for something, selling during dramatic falls in price is not a good idea.  It's the opposite of the mantra "buy low, sell high".  It's often worth waiting until the next bull market (probably a long time).
Altcoins' long-term trends are often similar to BTC, so probably not, but it depends what you're investing in.
Yep.  HODL it.