However, she said setting the targets had highlighted problems in the way poverty was measured.
Poverty measures were based on the Household Incomes Survey but Ardern said she was advised those numbers were not accurate because the sample sizes were too small and underestimated the numbers of sole parents.
That meant the data Treasury used to estimate the impact of policies was "a bit ropy".
"So my current guesstimate is poverty numbers will get worse before they get better because we are going to do a larger sample size next time round."
Ardern said much of the first three-year goal would be achieved through Labour's Families Package which kicks in on July 1. That includes increases to Working for Families, the Accommodation Supplement and measures such as a $60 a week payment for newborn babies and subsidies for winter heating for superannuitants and beneficiaries.
That is expected to benefit 384,000 families – and Treasury has calculated it will lift 64,000 children out of poverty.
"That is the centrepiece, it is $5.5 billion worth of investment in low and middle-income families. People won't be surprised that is the centrepiece, it's one of the largest pieces of reform we've seen in probably a decade in that area and a big boost."
However, she said non-income factors included the cost of healthcare, housing, the cost of food and other household costs.
In January, Ardern set the 10-year targets of more than halving child poverty but had held off on the shorter term 3-year goals until Treasury re-assessed the impact of the Families Package.
Treasury had earlier overstated the figures for which it blamed a coding error but issued its new calculations two weeks ago.
The three-year targets are:
• 70,000 fewer children living in low-income households before housing costs are included (a 6 per cent drop)
• 40,000 fewer children in low-income households after housing costs are included (a 4 per cent drop)
• 30,000 fewer children living in material hardship (without basics, healthy food etc) (a 3 per cent drop)
Low-income households are those on less than half the median income.