The three, from the southern city of Seville, made the announcement late Thursday after meeting Spanish Foreign Minister Alfonso Dastis in Madrid.
"We've received commitment from the minister that we will get all the necessary consular help to face this process," said Manuel Blanco, one of the accused.
In a tweet, Dastis described the trio as having done "rescue and humanitarian aid" work.
After their meeting, the firefighters told reporters they were detained in early 2016 while volunteering for Proem-Aid, a Spanish group of emergency professionals that helps people in distress in the Mediterranean.
Greek coastguards arrested them on the island of Lesbos -- the destination of thousands of migrants making the perilous sea crossing from Turkey.
"Just like we go out on a rescue mission if there is an earthquake, we were rescuing people who needed help," said Blanco.
"In this case, they were at sea, trying to reach Europe's coasts," he added.
"We went on a rescue mission that night in really cold conditions, in winter. But there was no rescue as we didn't find the boat."
On their return, the firefighters were stopped by Greek coastguards.
Blanco said no migrants were on their boat, but prosecutors accuse them of "trying to facilitate the entry of illegal migrants".
The firefighters are due to stand trial on May 7 in Mytilene, the capital of Lesbos.
Apart from the three Spaniards, at least one member of another similar NGO, Team Humanity, will stand trial on May 7, according to the group's Facebook page.
Some 5,100 migrants died in 2016 crossing the Mediterranean, according to the International Organization for Migration.
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