The governors of the only three states now accepting low-level nuclear wastes today gave the federal government three weeks to tighten the enforecment of safety rules on waste shipment, saying that controls must be "drastically improved" if their states are to continue receiving the radioactive materials.

Govs. Dixy Lee Ray of Washington, Richard Riley of South carolina and Robert List of Nevada, in a letter to the Nuclear Regulatory Commission and the Department of Transportation, said they were acting jointly because of "the serious and repeated disregard" of existing regulations.

The three governors said a blueprint for tightened safety practices must be delivered to them by August 1, to be put in effect a month later, or they will move on their own to "protect the health and safety of our citizens."

The letter did not spell out what actions the governors might take, but Riley noted that he had cut off shipments from the stricken Three Mile Island plan and that List had imposed a temporary blockade on shipments to his state.

Riley, Ray and List issued their ultimatum as the National Governors Association wound up its meeting here with a spirited battle over energy tax policy. After heated debate, the governors said that proceeds of the Carter administration's proposed oil company "windfall profits" tax should be used for conservation practices and relief to hard-hit homeowners, as well for development of new energy sources.

The resolution was approved by 19 of the 28 governors remaining at the end of the three-day session -- the bare two-thirds needed for adopting a policy resolution. Earlier, a move to ask Congress to let the oil companies escape the tax by plowing all their earnings into exploration failed by a 14-to-12 vote.

In a preview of the coming congressional debate, Republicans and some oil-state Democrats argued that the profits resulting from decontrol of domestic oil prices should be devoted to expanded energy production, either by allowing the oil compaines to retain reinvested profits or by financing government energy efforts from a tax on the profits.

"I concede there is a humanitarian side to the issue," Taxas Republican William Clements said. "But relief for the homeowners is a completely separate question."

However, Arizona Democrat Bruce Babbitt said that the oil companies would use he extra profits to "buy up clothing stores and circuses, not to develop new energy sources.

The resolution, as approved, puts the governors on record as supporting the Carter administration's proposal for phased decontrol of oil and endorsing a plowback credit for the oil companies, but includes as beneficiaries of the windfall profits tax energy conservation and "energy emergency impact assistance prorams" for hardhit users of fuel oil.

In another action today, the governors asked Congress for a law setting an 80-ton load limit and 60-foot maximum length for trucks on interstate highways -- effectively raising the limits in 11 states. The administration is seeking temporary higher limits as parts of the comprosmise that ended an independent truckers' strike, but the governors said the limits should be lifted permanently as an energy conservation measure.

In their letter to the NRC and Transportation Department, Ray, Riley and List complained about "the total lack of corrective measures" to reduce the hazards involved in the shipment of nuclear wastes.

Riley said in an interview that he and his colleagues were "out of patience" with federal officals and had decided on the joint letter as "the only way to force some action."

They said the plan to be delivered by August. 1 must include the dispatch of teams of inspectors to the producers and collectors of nuclear wastes and "the collectors of nuclear wastes and "the consistent and uncompromising enforcement of sanctions imposed whenever violations are discovered."

For the longer term, the three governors also called for a plan for regional wastes-disposal centers. Riley said 80 percent of the nation's nuclear wastes are being dumped or shipped within his state's borders, and he complained that the federal government has taken no effective steps to force states generating nuclear wastes to provide for disposal.

The disposal of low-level radioactive material has become a major point in the broader national debate over the future of nuclear power.