Experienced Ospreys centre Sonny Parker, who was in the national set-up's summer training camp, is called up to replace James, and young Dragons prop Jamie Corsi tours in place of Thomas.
A final place in the backs remains vacant as Jenkins promises to keep a close eye on several players involved in Magners League rugby this weekend.
Thomas (ankle) and James (knee) have both suffered injuries forcing them out of the tour and Luscombe has withdrawn in order to return to his family home in South Africa for personal reasons.
"Dafydd (James) has a cartilage problem which requires surgery followed by a short period of recovery, forcing him to miss the tour, but giving him a chance to return this summer," said Wales Team Physio Mark Davies.
"Rhys (Thomas) will be in plaster for three weeks after damaging ankle ligaments and will then need a further three to five weeks recovery."
Jenkins admits losing the trio is a blow to his plans, but he has every confidence in the highly experienced Parker and 19-year-old front row prospect Corsi.
"We wanted to have a good look at all three players and it is disappointing to lose them at this stage," said Jenkins.
"We were all looking forward to working with Rhys who has been out of action for some time with another injury and Daf was also chomping at the bit to get on the plane, but the hope is that all three will still have a chance to show us their worth before we finalise our World Cup squad.
"Sonny is a class act and we are lucky to be able to call on someone of his quality, experience and undoubted talent to boost the squad. He was overjoyed to take the call and is very much looking forward to touring.
"Jamie is a young prop who has caught the eye at youth level. He has a big future in the game and we are both confident and comfortable that he will take the step up and benefit from the opportunity presented to him.
"He has already represented Wales at U16, U18, U19 and U20 levels and was a member of the Wales U19 Grand Slam winning team in the 2005-2006 season.
"We will make a final call on who will replace Hal after watching regional matches this weekend."
Sonny PARKER (OSPREYS)
DoB: 27.08.77. 6'1" 1.86m. 14st 4lbs / 91kgs
International record: 21 caps (W 11, L 10; 25 pts - 5T)
Jamie CORSI (NEWPORT/NEWPORT GWENT DRAGONS)
DoB: 30.12.87. 6'3" 1.91m. 17.4lbs /110kgs
International record: Uncapped