Members of the Rail, Maritime and Transport union at Southern, Merseyrail and Arriva Rail North will walk out on Friday and Monday in bitter rows over the role of guards and driver-only trains.
The Merseyrail workers will also strike on Sunday.
The union has written to Transport Secretary Chris Grayling saying it believes the minister has approved a deal between Southern's owners Govia Thameslink Railway and the drivers' union Aslef which would affect its own dispute. Aslef insisted no deal had been made.
In the letter to the minister, RMT general secretary Mick Cash said: "I am hearing reports that a deal has been agreed with Aslef and GTR regarding the driver-only operation dispute.
"Any deal reached with Aslef will have had to be approved by yourself as you are directing this dispute and we understand you have agreed the deal."
Mr Cash said: "We cannot have a situation where RMT guards and driver members are being forced to take further industrial action on Southern from Friday while the company, with we assume the approval of the Transport Secretary, are off cutting reported deals which have a clear relevance to our dispute.
"That is a nonsense and reinforces RMT's call for round-table discussions.
"It is imperative that Mr Grayling and his contractors, GTR, give full disclosure to the deal that has been put together and engage with us in further face-to-face talks aimed at reaching a safe and sustainable solution to this long-running dispute."
Aslef said there was no deal on the dispute or other associated issues that are part of talks with the company.
General secretary Mick Whelan said: "We would look at the responses of the other parties involved and then consider our position which has been consistent due to the litigation and the associated decisions impacting on Aslef."
A spokesman for Southern said: "Discussions with Aslef are ongoing and we continue to work hard to reach a resolution to this long-running and unnecessary dispute."
The Southern dispute started more than 16 months ago, with the RMT taking over 30 days of strike action.