Claimed my mother this year as my dependent she has been living with me for the lady few years can I go back and claim her for those years too

Yes. You may file amended returns for 3 years back.  2015 must be filed by April 15, 2019.

See dependent rules at 

https://turbotax.intuit.com/tax-tools/tax-tips/Family/Rules-for-Claiming-a-Dependent-on-Your-Tax-Return/INF12139.html

on a separate file or on the same one which is in process

Each year is a separate filing. You do not include anything about years 2015-17 on your 2018 tax return filing.

How to amend <a rel="nofollow" target="_blank" href="https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed">https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed</a>

You will file a separate amended return for each of the years 2015, 2016 and 2017.