Did not paid any advance tax till now but now i fall under tax slab

I am freelancer and Since all my clients are outside India i don't need to register under GST, am i right?

I wasn't expecting my income to be more than 5 Lacs so i did not paid any advance tax.

But in last couple of month I made significant income and my total expected annual income is now 6 Lacs.

I did not paid any advance tax for all three previous installments, Now 15 march is last date for advance tax.

Can i still pay my advance tax? What about advance tax installments that I missed?

What is penalty for not paying advance tax on previous 3 installments?