I have lowered the number of allowances I am claiming on W4 from 5 to 3 on Mar 31, 2019. Do I still need to make estimated payments in 2019?

[JohnW15]

You may need to. 

The only way to tell, really, is to check your next pay statement, and project both your income and your withholding out to the end of the year, and adding in the income and withholding from before you made the change.  Then, add in any other income you might be getting.

That way, you'll have a rough idea of whether you'll have enough withheld to offset your taxes for the year.

Bear in mind, also, that if you're eligible for any tax credits next year -- child credit, child care credit, EIC, etc. -- that those should offset any tax due, as well.