Forgot to take itc in f.y. 2017-18

yes u can claim itc via showing the amount in gstr_3b in eligible itc 

you claim Till Sep

FY 17 -18 Can be claimed till Sep of next fu

Yes can claim your missed out ITC in next GSTR3B returns.

Note: ITC can be claimed only within one year from the date of issue of tax invoice relating to supply. In any other case, the last date to claim ITC is earlier of the two below:

Before the filing of valid return for the month of September following the end of FY to which such invoice is related, or Before the filing of the annual return