Questar Corp. overcame a $3.8 million third-quarter loss for its subsidiary, Mountain Fuel Supply, and posted a net income of $1.65 million, or 9 cents per share, for the third quarter, up from $782,000 or 1 cent per share for the same period last year. Questar said increased natural gas production from recently acquired properties and completed wells, as well as higher oil prices, helped boost its earnings.

For the first nine months of 1990, Questar earned $35.25 million, or $1.77 per share, up from $33.68 million for the first three quarters of 1989. Average shares outstanding were 19.5 million as of Sept. 30, up from 19.3 million a year ago.Questar's exploration and production operations increased their combined third quarter net income 81 percent from $2.08 million in 1989 to $3.77 million for the most recent quarter. Nine-month earnings were 55 percent higher, $11,259,000 in 1990 compared with $7,276,000 a year earlier.

The average sales price of the company's natural gas was flat at $1.86 per thousand cubic feet.

The spokesman said Questar has agreed to sell some 2,200 barrels of oil per day for a six-month period beginning Nov. 1, 1990, at an average price of more than $35 per barrel.