I believe one of the three bodies was that of First Class passenger Thomson Beattie. Who were the other two?
Yes you're correct that one of the bodies was Thomson Beattie. We know it was definitely three male passengers. It has been speculated that one of the bodies was Arthur O'Keefe. From O'Keefe's biography:
"On the night of the sinking it is speculated that O'Keefe was one of the men who managed to pull themselves aboard the waterlogged collapsible lifeboat A and was the man that Norwegian passenger Olaus Abelseth tried to help. Abelseth had shared a carriage on the boat train to Southampton with a New Jersey man who he later encountered in boat A, lying freezing, so he propped him up and attempted to revive him, telling him to brace himself and that a ship was coming. The man would say "leave me be," and "who are you?" before he died of exposure. The body of this man was left in the boat when Officer Lowe arrived to transfer her survivors. A month after the sinking the Oceanic came across the drifting collapsible A, the three bodies were recovered and buried at sea."
However, a passenger aboard the Oceanic by the name of Sir Shane Leslie, described the scene:
‘The sea was calm at noon when the watch called out that something could be seen floating ahead. The ship slowed down and it was apparent that the object was an open ship’s lifeboat floating in mid Atlantic. What was horrifying is that it contained three prostrate figures. Orders from the bridge dispatched a lifeboat with an officer and a medical officer. What followed was ghastly. Two sailors could be seen, their hair bleached by exposure to sun and salt, and a third figure, wearing evening dress, flat on the benches. All three were dead and the bodies had been tossing on the Atlantic swell under the open sky ever since it had seen the greatest of ocean liners sink. The three bodies were sewn into canvas bags with a steel bar at the end of each. Then one after the other the bodies were draped in the Union Jack, the burial service was read, and they splashed into the sea.’
An account written on May 13, 1912 by an unknown passenger who was aboard the Oceanic, describes it differently:
"I crossed the Atlantic one month after the Titanic catastrophe. We picked up one of the lifeboats with two n****r-like unrecognisable corpses of a passenger in evening dress and two firemen, wedged below the seats.
“The arms came off in the hands of the Oceanic boarding officer.
“Women’s rings were found.”
“The bodies were buried and the prayer service read. The lifeboat then hauled on to our deck when I cut this piece out of the boat covering.”
In Elin Lindell's biography:
"Despite briefly reaching Collapsible A Elin died. Edvard held her wedding ring in his hand when she drowned, the ring came to be on the bottom of collapsible A that was not taken up by Carpathia
. It was found by crew from the Oceanic 13 May 1912".
While in her husband's bio, it is less clear:
"Once the ship went under Wennerström and Lindell climbed into the boat. Wennerström saw Mrs Lindell in the water and grabbed her hand. Weakened by the cold he was unable to assist her further and after a while she drifted away. During the night according to Wennerström "Edvard's hair turned all gray in lesser time than 30 minutes". He died soon afterwards and lost Gerda's wedding ring which he had been holding. Neither his or Gerdas bodies were found, he was probably lowered overboard to make the unstable boat lighter."
Since Lindell apparently perished soon after his wife, there obviously would have been no need to keep his body on board the boat. Furthermore, his body was never found so that eliminates Lindell as being one of the bodies in the boat. That leaves O'Keefe being one of the bodies and possibly a fireman.