WASHINGTON — Lawyers for three condemned Oklahoma prisoners who claimed that the three-drug combination that could be used to execute them risked causing unconstitutional pain and suffering ran into skepticism from conservative members of the Supreme Court on Wednesday.

The prisoners argued that the sedative midazolam, which was involved in three prolonged and apparently painful executions last year, could not reliably produce a state of deep unconsciousness before other, severely painful drugs were injected. They asked that lower court rulings permitting the use of the drug in executions be overturned.

But several of the conservative justices questioned whether the evidence warranted a reversal and, more broadly, expressed exasperation with shortages of more proven drugs that they said had been caused by opponents of capital punishment.

“Let’s be honest about what’s going on here,” Justice Samuel A. Alito Jr. said. “Executions can be carried out painlessly.”