Iran's state-controlled video service Iran Press states that the military exercise is the first of its kind between the three countries since the 1979 revolution.
The exercise started on Friday and will last for four days. However, it is unclear exactly where the exercise will be carried out, and exactly how many and which vessels will participate from the three countries.
According to Iran's Deputy Marine Chief Gholamreza Tahani, the exercise is conducted in a 17,000-square-kilometer area in the northern Indian Ocean. His boss, the Deputy Admiral Hossein Khanzadi, has previously said that the vessels leave from Iran's port city of Chahbahar in the Gulf of Oman near the Pakistan border, reports the AP news agency.
The Gulf of Oman is a strategically important fairway as it is directly adjacent to the Hormuz Strait, where one fifth of the world's oil passes out from the Persian Gulf.
"The message of this exercise is peace, friendship and lasting security through cooperation and unity ... and its results will show that Iran cannot be isolated," Admiral Gholamreza Tahani said on state television, according to British news agency Reuters.
On Friday, there are no exact details of Iranian, Russian or Chinese submarines participating in the exercise. Iran has a strong navy with a total of 21 submarines.
Russian state-controlled news agency Tass states on Friday that the naval exercise will be carried out in the Gulf of Oman and that a group of Russian military vessels will participate, including three vessels from the Russian Baltic Sea Navy - the robot frigate Yaroslav Mudry and two maintenance vessels.
According to Tass, an agreement was made on the trilateral military exercise last summer at the Russian Baltic Sea Navy's annual parade in St. Petersburg. Russia states that, among other things, they should practice exempting ships that are hijacked by pirates.
The spokesman for China's Ministry of Defense has said that China will participate with the robot fighter Xining and that the exercise is a normal exchange between the three countries' defense forces.
"It is not necessarily linked to the regional situation," said the Chinese Ministry of Defense spokesman, Reuters reports.
Tensions between Iran and the United States have risen sharply since US President Donald Trump in 2018 stated that the United States signed the international nuclear agreement signed with Iran in 2015.
In the summer of 2019, Iran seized several European oil tankers, including the Swedish-owned Stena Impero. There have also been wars of war between the US and Iran as both countries stated that they should have destroyed each other's military drones.
Then the United States Navy also stated that they expanded their military presence in the snake area.