The action, which involves Southern, Merseyrail and Arriva Trains North, is part of an ongoing dispute over staffing levels and the role of conductors across the industry.
The strikes coincide with the Grand National at Aintree, the biggest horse race of the year that is attended by tens of thousands of people – many travelling by train.
Merseyrail is expecting drivers in the Aslef union to refuse to cross RMT picket lines, so a team of managers is being drafted in to provide a service between Liverpool city centre and Aintree before and after the race.
A train will run every seven-and-a-half minutes at key times, but fewer or no trains on other parts of the network.
Jan Chaudhry-van der Velde, managing director Merseyrail, said: "Despite the RMT's best efforts to scupper this iconic event and create misery for the tens of thousands of people who look forward to it all year, we've developed a timetable aimed at getting as many people between central Liverpool and Aintree as we normally do on Grand National day."
Arriva Trains north said its services will be "significantly" reduced, with the last trains leaving Liverpool Lime Street between 7.02pm and 8.16pm.
The operator said it has added six trains to its Liverpool Lime Street service on Saturday evening to help racegoers travel home.
Workers at the three rail companies are said to be "solidly supporting" strikes amid the bitter rows, according to Mick Cash, general secretary of the RMT.
He said: "Support for all three strikes today is rock solid, with pickets out in force and morale high as our members on Southern, Northern and Merseyrail continue the fight for rail safety.
"It is down to all three companies to now get round the table with union negotiators and reach settlements based on the guarantee of a guard and a safe railway for all."
The Merseyrail and Arriva disputes are over new trains coming into service in 2020 which will be driver-only.
The Southern row is now a year old, with the RMT taking its 31st day of strike action on Saturday.
The union will parade an advertising billboard through Liverpool and Aintree explaining why the RMT is taking strike action to defend safety-critical guards.
The RMT is demanding an inquiry and a halt to the introduction of driver-only services because of safety implications.
Mr Cash said: "It is well established that once the guarantee of a guard is withdrawn then disabled passengers are disadvantaged because they can no longer be assured of being able to turn up and get on or off the train at unstaffed stations."
The Transport Salaried Staffs Association said in a message to its members at Merseyrail: "TSSA members are advised to carry out their normal duties and attend for work at their normal times of work and for their normal rostered shifts.
"Members should decline to change their rostered hours of duty except in circumstances where their contract of employment allows their employer to change them."
Southern is aiming to have a near-normal service during the strike, saying nine out of 10 trains ran during the last walkout in March.
The company announced it planned to run a normal train service on all but one of its routes.
Passenger services director Angie Doll said: "We're now running a near-normal service on RMT strike days, so fewer people are being affected by the RMT's action. We plan to provide almost our entire normal Saturday timetable."
No Southern services will operate between Clapham Junction and Milton Keynes Central via Kensington Olympia/Watford Junction.