Three congressmen have called on the Federal Trade Commission to investigate allegations of antitrust violations by Shell Oil Co. in the sale of wholesale gasoline.

At the same time, one of the three, Rep. John E. Moss (D-Calif.), asked the Department of Energy to "rethink" its position on the deregulation of motor gasoline. Moss is chairman of the oversight subcommittee of the House Commerce Committee.

Moss, and Representatives Toby Moffett (D-Conn.) and Andrew Maguire (D-N.J.) have written FTC chairman Michael Pertschuk with allegations that "Shell is requiring franchise-owned gas stations to purchase a fixed amount of leaded gasoline in order to purchase a lesser amount of unleaded gasoline."

All three say they are concerned that Shell's alleged activities, which they say would, if true, constitute antitrust violations, are also further complicating another issue: the deregulation of motor gas.

Many environmental groups, and the Environmental Protection Agency, have criticized the DOE for attempting to deregulate the price of gasoline. They contend that such a move would cause the price of unleaded gasoline to soar much higher than leaded, which is cheaper, resulting in many motorists switching from the cleaner unleaded to leaded gas, ruining the catalytic converters in the cars and causing increased pollution.

If it is forcing dealers to take more leaded gasoline if they want unleaded, Shell would be virtually making the dealers sell the leaded cheaper in an attempt to get rid of it, and subsequently making it more attractive for motorists to switch fuels.

In addition, Moss points out, there are spot shortages of unleaded gasoline in "several areas of the country."

Moss said "the shortages are increasing at the very time the DOE is pushing ahead with plans for gasoline decontrol."

Moss' subcommittee held hearings in July, in which it was revealed that the DOE had withheld an internal study that indicated a potential gasoline shortage in 1980.