The protest-hit Sustainable Farming Scheme (SFS) was introduced earlier this year, after almost a decade in development.
Payments with a greener focus replace the subsidies farms had received before Brexit based on how much land they had.
Those who sign up have to agree to a checklist of requirements - including managing at least 10% of their land as habitat.
The budget announcement is being made on the opening day of the Royal Welsh Agricultural Show in Llanelwedd.
Caryl Davies, a beef and sheep farmer from Eglwyswrw, Pembrokeshire, said her family had decided to join the new scheme.
"There are ways that farming and nature can work together, it'll improve the soil and the crops we're growing," she said.
The fact that the budget was being ring-fenced over several years would "give people a bit more confidence they've made the right decision," she said.
"A three-year plan gives you some certainty about what you're going to get," agreed retired livestock auctioneer Alwyn Jones, showing his pedigree Welsh Black cattle.
But the money would not go far, he said, as costs have gone up and "everything has gone dearer".
While a three-year budget is welcome, "five years would be better and a bit more money obviously," added Stephen Roberts from Denbigh.
NFU Cymru president Abi Reader said the industry would "really welcome the multi-annual funding".
"If we were still in the EU we would have a seven-year cycle so we're not quite there but at least we know for the next three years it's certain," she said.
"It would be really good to see a budgetary uplift," she added, but acknowledged that "in this economic climate that is going to be a real challenge".
Farmers' Union of Wales president Ian Rickman said recent years had brought significant financial uncertainty for the farming sector in Wales, making it "increasingly difficult for businesses to plan with confidence and make informed long-term financial decisions".
"The FUW has consistently called for a five-year funding framework, so today's commitment to provide certainty until March 2030 is a welcomed relief for the family farms of Wales," he said.
Environment groups said more funding would be needed to help farmers - who manage 90% of the Welsh landscape - restore nature and tackle climate change.
"For many people, the future of farming can feel like a distant policy debate. But the choices made through the SFS will shape the rivers we swim in, the wildlife we see, the food we eat and the way Welsh communities cope with floods, drought and a changing climate," explained Rachel Sharp of Wildlife Trusts Wales.
The charity, alongside the RSPB and National Trust, had called for a doubling of the current budget, she noted.
"Sticking with the same budget level is disappointing," she said, urging ministers to introduce full details of the scheme's optional and collaborative funding pots quickly.
A couple of the optional choices that farms can apply for as part of the SFS were announced by the government as part of the budget announcement.
Funding for farms to go organic will be available from the end of the month, a move welcomed by the Soil Association Cymru, who said it was "good news for Welsh food and farming".
