Re: Where is the first half of Supergirl, Season 3?
My advice: watch episodes of this series as soon as you can if you are at all interested. The CW maintenance is horrid even for recent episodes, as proven by the SD episode 315 incident. It was missing and was never restored after weeks of me complaining and now 315 is expired for HD and online viewing, so it is moot now.
(I tried complaining directly to the CW via their webpage but it has a non-working CAPTCHA feature that prevents its use.)
So it is a ramshackle provider. You should hope that your favorite shows in the future appear on different networks, not the CW.