Gabriel Diya, 52, his daughter, nine-year-old Comfort and son Praise-Emmanuel, 16, drowned in a pool at Club La Costa World in Fuengirola while on holiday on Christmas Eve.
A judge permitted the family to bring in their own team of investigators after the deaths were originally ruled to be a "simple accident" as no malfunction of the pool could be found and the issue was a "lack of swimming expertise".
Comfort and Praise-Emmanuel got into difficulties in the deepest part of the pool and Gabriel dived in to help but all three died in the incident.
On Monday, a diver could be seen in the pool as the search for further evidence began. An engineer will also inspect the pool.
The latest investigation will review the pool's pump and motor, in addition to analysing water circulation, suction and drains.
Additionally, the resort must also hand over CCTV footage on the day of the incident.
A report by three investigating teams also revealed further details of potentially contributing factors to the deaths.
The report showed:
The pool temperature was around 16C, according to police divers. They say this is relatively low for swimming.
A worker at the complex who jumped in the water to help found it hard to get out of the pool despite being a strong swimmer and surfer.
Comfort’s swimming cap, retrieved from the pool, had her handwritten name inside it. She was also wearing swimming goggles, details, the family’s lawyer argues, that show she was used to swimming.
Swimming pool expert Allen Wilson
Swimming pool expert Allen Wilson is backing concerns from the family that the pool may have had a fault, raising concerns about the single drainage outlet at the bottom of the pool.
Mr Wilson told ITV News: "The problem if you have just one single drainage hole is entrapment.
"You've got a huge velocity of water going out that way; people will get dragged down, and they would not be able to escape from there.
"There's been numerous incidents all over the world resulting from excessive suction through the bottom outlet."
The owners of the resort have continued to work with all the relevant authorities.
A spokesperson for CLC World said: "We have cooperated fully and transparently with the authorities from the outset, giving every assistance to their investigations into this tragic accident.
"Naturally, we will continue to do so... we remain confident that there were no issues with the pool, its maintenance or its functioning which contributed to this tragedy."