0349 GMT April 24, 2019
Nabil Adeeb confirmed on Sunday that a judge had found the three, who worked for a group providing training in human rights issues, guilty of publishing fake reports or spying.
"The judge found Khalafalla Al-Afif and Midhat Hamdan guilty of publishing fake reports," media outlets quoted Adeeb as saying. "Mustafa Adam was found guilty of spying."
According to the lawyer, each activist was also fined 50,000 Sudanese pounds (about $7,460).
Adeeb has said that the defense team will still lodge an appeal.
Reports say the judge read the verdict at a packed hearing attended by Western diplomats and many other rights activists.
Al-Afif, Hamdan and Adam were arrested on May 23 last year after Sudanese security agents raided their office in the capital, Khartoum.
During the raid, security agents detained eight activists, but five were later released.
Global rights groups have regularly called for the release of the three, who worked for the Center for Training and Human Development, known as TRACKS.
Alice Mogwe of the International Federation for Human Rights (FIDH) said in a statement released ahead of Sunday's sentencing that the three "have become tragic symbols of the challenges faced by civil society in Sudan."
"The future of Sudan's civil society is at stake and there are reasons to express deep concerns."
FIDH says throughout their trial, the three were never presented with any evidence proving their guilt on the charges against them.
Sudan's powerful National Intelligence and Security Service (NISS) has been under fire for detaining opposition politicians and human rights workers.